Comment by Terr_

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