I don't know, I can think of plenty of rewards you can find that aren't straight cash.
"There's different kinds of treasure mate."
There are prizes all around you if you're wise enough to see them!
More seriously, this is one of those worldbuilding things that I personally like to think a lot about. Why
would there be treasures in the places the party goes? My answers thus far have been:
Long-lost ruin or long-buried site recently revealed/rediscovered, so the party are among the first to see it in thousands of years
Planar location difficult to escape from, so it has a lot of loot that no one has been able to pull out before
Long-held enemy strongholds, so any treasures there were
stored there purposefully
Recently-built enemy encampments, so any recovered treasure is likely the spoils of "war" (really, just minor conflicts)
Straight-up payment for services rendered, whether private individuals or dignitaries drawing from the royal vaults
Trade goods! Haven't used this much as treasure per se, but it's implicitly in the background
For one of the other settings that exists only in my head--a "post-post apocalypse" world, where society is
just beginning to recover the heights of its ancient past--the answer is very simply that there are TONS of ruins and crypts and hidey-holes from the Dark Age (also known as the Ice Age), where ancient kings were buried and the knowledge of the ancients is kept. The PCs are thus part of the first (or perhaps second) generation of heroic adventurers both brave and
equipped enough to actually survive delving into these places. Places that the ancients usually didn't
want anyone delving--sometimes for good reasons, often just "stay away from our
stuff."
