Depends on the type of monster. Dragons are notorious treasure hoarders. Gold golems are literally made of gold, so no mystery there. Orcs, goblins, and the like can basically be treated like humans. You'd expect them to carry money. Other monsters may have various other reasons for having gold on or in them.

Another possibility is it's just a streamlined version of what some other games do where monsters parts can be traded for money. Essentially it just assumes your character does the trading for you so you don't have to waste your time.