Not a believer, just playing devil’s advocate:

1.) You’re not likely to find a carcass unless something has killed it. Sick and dying animal tend to hide. Adult bigfoot probably have no natural predators, and they likely keep their young very close when they can and well hidden (in trees? caves?) when they can’t.

2.) When an animal is killed, whatever killed it is probably eating a good portion of the meat, and whatever isn’t eaten is picked up by scavengers, maggots, and rot. Even bones are eaten by many animals, and what isn’t eaten wouldn’t necessarily be noteworthy to the average person (although that does raise the question of why no expert has found any).

3.) It’s possible that bigfoot bury (or consume, as someone else suggested; never thought of that before) their dead. They almost certainly have death rituals as elephants and chimpanzees do, and would probably be aware of the scavengers and disease that corpses bring. One problem with this tho: bigfoot are typically reported as traveling on their own or perhaps in very small groups, whereas elephants and chimps travel in large groups. Perhaps witnesses are unaware of the others hiding around them, or perhaps bigfoot only travel alone, but shelter in large groups (see my other point about hiding when sick or dying). Albert Ostman said he was held captive by a family of four, which isn’t very large but does suggest that perhaps they live in larger groups than is typically reported.

4.) Perhaps someone has come across a body, but either didn’t see it hidden in the brush, didn’t recognize it as a bigfoot (see point #2), or didn’t know what to do about and moved on, keeping it to themselves.

I don’t think this necessarily is a satisfactory set of explanations, but I do think people overestimate how easy finding a body would be.