|The short answer: Because "we" have never looked for these kinds of remains.|
Nobody Looks for Bigfoot Remains
No serious work has ever been done to look for remains of surviving wood apes in areas where they are rumored to reside. No one should expect remains of such an elusive species to be found, collected and identified without some effort.
Very few remains of ancient wood apes have ever been found in Asia, where they were much more abundant. Millions of gigantos (a branch of the wood ape line) lived and died in Asia over the ages. All the remaining physical evidence we have of them could fit into a few shoe boxes. Fossils of any land animal are very rare.
Remains do not become fossilized very often, but unless that happens, all the remains will, in time, become completely reabsorbed into the ecosystem. There would be remains of animals everywhere if remains were not naturally recycled, including bones and teeth.
Fossils or preserved bones of wood apes may exist in the Americas, but they will be exceedingly rare, because these animals are rare to begin with, and only a tiny fraction of that population will die in locations and soils that will preserve bones somehow. Odds are slim at best that any bones (which are normally fragmentary) will be found, collected and identified unless a focused effort is made to look for them. Until efforts are made in many places, over a long period of time, no one should be scratching their head wondering why "we" don't have any physical remains.