There are extremely good security reasons for it.

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].

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[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

With very few exceptions, if there is something in you that isn't you then you would be better off if it was dead.

In this case nature was just indifferent to transplants as they weren't a thing even 100 years ago.

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.

I'm a kidney transplant patient myself. Your cells have HLA markers to distinguish your own cells from foreign cells. That is to prevent your immune cells from attacking your own body. This also makes it harder to find a organ match.

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.

There is zero inherent way to tell a malicious group of proteins and RNA or cells or biological process from a non-malicious once. They are both the same exact activity. Your body is killing cells all the time, so even cell death isn't a certain indicator (though is an indicator in the innate immune system and cells dying the "wrong" way will activate a response)

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

The way your body detects foreign cells is the same way your body detects foreign cells. It detects anything that doesn't have your, specific, randomly-generated at conception, friend-or-foe markers.

To deal with:

1. Bacteria

2. Parasites

3. Cancer

In all these cases cells that are "other than your own" are in your system and need to be eliminated, in the case of cancer it's your own cells but with significant mutations, as far as I can tell there is always an immune response although in some cases it is actually counter productive.