Not necessarily. Many battlefields were left for Mother Nature’s helpers to clean up.Same if disease wiped out a group. No graves and eventually evidence decays.
Yes, exactly. Considering the length of time humans are agreed to have been in the Americas (and it’s probably a lot longer than that), the dearth of Precolumbian remains illustrates your point.
The other practice (found among some Plains tribes, and in Asia among Zoroastrians) is excarnation, which is, basically, feeding the birds. Whatever is left of after defleshing and rotting turns to dust due to exposure to the elements.