Argthtjtr

Because the blood level midichlorian equilibrium point is determined by your body's biochemistry. You can't alter this by adding more midichlorians. All that does is cause a temporary imbalance (way too short for your body to make use of it) that your body quickly fixes. Excess midichlorian gets filtered out by the kidneys and ends up down the drain. Literally.

Midichlorians are not what gives you the force, they are a by-product of being strong in the force. Excess psychic energy goes into making the midichlrians. All that blood is wasted because the midichlorians come from the force, not the force from midichlorians.

Homeostasis. Your body maintains an equilibrium, like blood or temperature

A normal human has around 5 litres of blood in their body. If you transfer a litter of to your body (and not die), would now you have 6 litres? Well... maybe, but not for much longer.

Or temperature, you are at 36℃, if you increase/decreases your body heat from your surrounding, will it stay in that forever? Of your not. Your body maintains a level of temperature, by warming up or cooling down at certain thresholds.

Your blood amount is also determined by your body, not your blood transfusions, if you have a lack of blood, your body will produce more to compensate it, and if you have too much, your body will dispatch the blood's water from kidneys, and red cells' will be recycled into bilirubin (yellow) in the spleen and that dispatched from duodenum as stercobilin (brown). Interesting transformation of colours, right?

Midichlorians are like blood, and a lot more things in our body, like hair, adipose tissue, temperature, etc. A transfusion may only increase the Midichlorians during a very short time (if the body is able in the first place to take advantage of them and don't die/get sick due an increased level), then the additional amount will be quickly destroyed/dispatch/recycled by your own body...

...Or maybe not, they aren't destroyed, but anyways they have a certain lifespan, so if you transfuse them to your body, they will work in you until they "get old" and die, like red cells (120 days). Then you should transfer more from the original source... or steal their Midichlorians's stem cells. That is up to you.

Because they are genetically foreign.

They immediately start to reject the body, and the body rejects them. Welcome to a force fight at the cellular level.

Immuno-suppressants allow the transfusion to succeed without ripping your body apart, but the downside is that they significantly hamper even your own ability to interact with the force, injecting extra barely raises you back to functioning.

Cloning ;)

Okay, genetically identical. Except that the Midichlorian's are already at saturation in your body, adding more of the same simply causes them to die off.

You might, through an ingenuous system of continuous transfusions keep yourself in a state of over-saturation. This would amp up the basic capacity for force manipulation (you will still need practice as with any fine motor control), but there are (semi-)permanent side-effects. The Midichlorian's learn that massive spikes occur, and reduce their overall numbers so as to survive long-term. You might recover to normal levels, it might be a permanent reduction.

Divergence

As the force wielder learns, the Midichlorians alter themselves physically. Even if they were cloned and successfully duplicated, reintroduction back into the body causes the equivalent of a theological argument between religious sects. At best nothing changes, at worst valuable skills (learning) are lost as the two sides duke it out, or the host dies.

In short you are extremely lucky to be alive.

Putting more fuel in your tank is not going to make your car drive any faster.

Midichlorians are a resource, but it's unclear how this resource is being activated. If, for example, the brain uses midichlorians in order to send out a psionic signal which connects to the Force; then there's a natural cap at your brainpower.

For example, let's say your villain's body actually generates less midichlorians than their brain can use. At this point, adding more midichlorians to their blood will in fact increase their brain's psionic output as it is no longer running at suboptimal efficiency due to lack of midichlorians.

Analogy: Your car will run better on a tank of gas than it will on fumes.

However, if the villain already generates enough midichlorians to saturate their brainpower at full capacity; then adding more midichlorians to their blood is not going to help, the bottleneck is their brain capacity.

Analogy: Putting even more fuel in your tank is not going to make your car drive any faster.

This creates the option for you to have the villain initially see some improvement, but then very quickly plateau.

Note

I suggested that midichlorians are consumed by the brain and regenerated by the body; but the same would work if midichlorians were considered a catalyst which does not get consumed (nor regenerated). The brain can only use so many midichlorians at one time.

Pure math.

If you mix two liquids together, whatever makes them special doesn't get stronger. You get the average strength of both.

Say you mix a strong and a weak beer together, the strength will be in between the strength of the two.

You do need a bigger container though.

After all those transfusions, his midichlorian level is the average of all the people he transfused from.
His body will contain a bit more blood though.

This also dictates that he can never get stronger then the strongest Jedi he kidnaps. So unless he finds out a way to extract the midichlorians from the blood, he's out of luck.

A blood transfusion wouldn't change the number of midichlorians in your other cells and there's no reason to think that your red blood cells are particularly important to your sensitivity to the force...especially when compared to, say, your brain cells and your nerve cells which seem much more likely to mediate between your mind and the Force.

That being said it's actually pretty likely that such a villain would experience a boost in their ability to use the force. Combine the placebo effect with the Force and you could get some real action.

Obviously blood can only contain a certain density of midichlorians or it would clog up and ultimately become solid. Your body will naturally break down any excess of midichlorians and convert them into fat. This is what happened to Jabba the Hutt.

Midichlorians act as a mediator between the Force, the mind, and the body, all three acting in concert. Because of this, midichlorians aren't just sensitive to the force, but to the unique pecularities of each person's body chemistry and bioelectric field. These things change over time, of course, but it happens slowly enough that the organs which produce midichlorians can adapt as a person grows.

The shock of being suddenly removed from one body and transplanted into another is too quick for the midichlorians to adapt. Given enough time, they would indeed "learn" how to work with another person's body and mind, as theory might state. The problem is preventing the body from rejecting and excreting them before that happens: it just plain takes too long. This is not something your sith has mastered.