You don't see many knockoffs (Settlers of Zaramela, anyone?) because
Settlers has so official variants... I've seen a half-dozen unlicensed
computer versions of Settlers... all of which are knocking off core
Settlers, not any of the variants (Settlers Card Game, Settlers Dice
Game, Kids of Catan, Starfarers of Catan, Starship Catan)... but the
commonality is random resource production and player ability to trade
both with other players and with the bank, plus the use of specific
resources in specific combinations to build items which get you VP.
I'm certain other games have that same combo... I just haven't played
any.
I've played a couple games with random resource generation - and they
predate Catan. They didn't link it to *where* you built, tho...