Well, the bigfoot believers would say that Bigfoot is a rare critter; we don’t know how long it lives; and that nature has a way of destroying and scattering remains - and all of that would reduce the likelihood of finding remains. Or they would suggest that the Bigfeet take care of their dead in a primitive way similar to the way that we do.
Or they would ask, ‘how often have you found a dead bear carcass or skeleton in the woods?’
In my case four dead bears and two human bodies.
Once. Mostly eaten deer remains. It was gone by the next summer (though I did not dig around to look for remaining bones).