Another trope <he pointed us at tvtropes.org! gasp! infamy!> is the worn-out world. Instead of new and scantily-resourced, one could have an old landscape that had hosted many prior civilizations, each of which had done its level best to loot the soil, exhaust the mines, and use up the mana. This is a parallel trope to the "long-gone elder race" bit.

So maybe the first dozen times folks got up to the world-spanning societal level, it DID just take them a couple of thousand years. Then maybe your world has a tendency to kill off the bulk of the people and stomp them back to the stone age. Plague? Orbital wobble? Variable sun? Warfare? Another 2 to twenty thousand years of savagery (and bad weather? ice? spreading jungles? typhoons? voracious stainless-steel-eating radioactive microbes?) between civ-fall and new civ-rise could suffice to erase most traces. A civilization properly disposed (properly scattered) might for example on average spread iron from mine-able deposits to diffuse streaks of rust. If for example cities were mostly built along large rivers, an eon of flooding could scatter many civ traces (refined metals?) out to sea. If the erasing mechanism was ice ages, definitely glaciers could grind faded remains to bits.