Sabin_Stargem: This one could be explained. The blob most likely consumed an commoner at one point or another, who just happened to have 150gp in his coin purse. The body, bones, and coin purse were eaten away, but the gold pieces couldn't be digested. Unfortunately for the slime, it eventually encountered some adventurers and died shortly after.
Wishbone: Yes, but now extrapolate it to all the mobs you've ever killed in a hack'n'slash. How exactly does a wolf come into possession of 15 gold pieces, 20 arrows and a breastplate of +2 Strength? And where does it keep them?
Arrows comes from the level 0 NPC hunters that have tried to kill it, gold is found in its belly (from said hunters), armour comes from when the wolf killed a level 1 adventurer, and in its greed crawled into the armour to eat every last piece of the adventurer, but got so fat that it could not get out.
As far as money goes in CRPGs, I've found it easier to explain why you find it in such odd places if you view it as an abstraction. It represents everything that has a real barter value, but is light enough/has no other uses to you, for it to be unnecessary to add to your inventory. That explanation does suffer in case games actually feel the need to represent those kind of things.