We do do palliative surgeries if warranted but the intent there is not to cure, but to relieve some symptoms and transiently improve quality of life.

Please direct any specific questions you have about your sisters case directly to her doctors

As anatomy gets more complex, the process of getting it from "arbitrary heavily damaged state" to "functioning state" gets more complex too. And mammals are a bit more anatomically complex than placozoa.

If your entire body is a hollow sphere 4 cells thick, "repairing arbitrary damage" is very simple and natural. When you have bones, blood vessels, nerves, muscles and tendons, all wrapped in skin - all of which have to be restored correctly for a lost limb to function well? The gap between "just plug the holes" and "restore the function" grows, and the complexity of implementing usable regeneration goes up massively.

Humans can repair most of simple tissue-level damage well enough. The complexity equivalent of placozoan regeneration is in place. Rebuilding complex anatomy is what's often unimplemented. Seems like that is the part that requires some novel adaptations rather than simply not deactivating the mechanisms that are already there.

Megakaryocytes are primarily bone marrow cells.

Even if we assume your body has a foolproof is_same_species() test, your neighbors' cells may:

1. Be healthy, but have the wrong priorities or behavior, so that they start doing the wrong tasks or sending the wrong messages in the wrong places. [0]

2. Be healthy, but they're just a creepy flesh-mask a parasite has constructed around itself.

3. Be individually hijacked by viruses, and your body won't have the same baseline information to tell that they've deviated from your neighbor's standard.

4. They may be fully human, but gone feral into a transmissible tumor [1].

____

[0] This is actually a risk from pregnancy, as overambitious fetal cells leak into the mother.

[1] https://en.wikipedia.org/wiki/Clonally_transmissible_cancer

As others have explained, foreign cells usually meant trouble, so fighting them by default was a good choice. And there was no pressure to allow transplants specifically, because they didn't exist.

Also, (I think) you usually need a transplant later in life, after reproduction, so too late for evolution to affect it. So even if we had transplants for millions of years, it might not have made much difference either.

The reason I needed a kidney transplant is because of an auto disease called IgA Nephropathy where the IgA antibody has a small defect that causes the rest of the immune system to attack it and the whole mess clogs up the kidney filters over time.

The only useful signal is "This wasn't here yesterday". Your body generates B cells with a little sensor that has it's own special systems to have especially randomized sensor activity. Your body is generating random binding sites because there's no rules for what a hostile protein would look like.

Then it prunes these B cells by showing them a bunch of proteins your body is capable of producing, and killing any cell that signals for proteins your body normally has. This leaves you with a bunch of B cells that have random sensors tuned to only trigger for things that weren't in your body yesterday.

This is great, because it's a proactive measure against evolution. It's the core innovation of the adaptive immune system, and a massive advancement in biological self defense.

https://en.wikipedia.org/wiki/Germinal_center

Cells from someone else will have some different markers and proteins than found in your body. Human genetics are 99% the same yes, but there is massive diversity in the remainder.

Note that this constraint is not biological in nature! Any general purpose "programming" environment cannot differentiate between "Hostile" and "Benign" by function or structure alone! The body, like many smart cybersecurity groups, made an allowlist. Though the way that allowlist is made is fascinating