Yup, it blew my mind when I first learned about it.
Back in high school, I did a kind of medical jobs class that supposedly prepped us for entry into medical training, to help decide if or was right for us, and which fields we might go into. There was more than that, but that was the basic idea.
The last year of the class was going out and playing tag-along on various jobs. Nursing, radiology, pathology, dentistry, whatever.
One of the things we got to go to was a transplant unit. Finding out that kidneys (usually) stay in was kinda crazy because the obvious thing is that they’re diseased, maybe dying, so why would you leave them in, wouldn’t that cause trouble down the road?