Setting aside the religion and societal aspects, I think it's just.... plain taboo. It's way up there with murder and desecrating the dead. Our brains are just wired to feel bad with just looking at the dead. What more if they did that to themselves? Suicide feels unnatural and assisted suicide unconventional. At least, that how I feel before I was younger and too religious. I reckon fellow catholics felt that way.
Maybe people also use religion to justify their hate for it. That's what they do to other things anyway.
It doesnt help the suffering though and their body autonomy still denied. But hey atleast they're not "uncomfortable" right?
Tldr: monkey brain hate death ungabunga only food and sex