Simplest; The goblins, and wizard society just do not approve of outright theft, even from muggles, and there are magics that will reliably mark stolen goods. So if you want to come up with gold by the tonne, you need to either actually engage muggles in trade (eewww) or go hunting for treasure with no (living) owners.

More amusingly: I am not at all sure competent wizards have much need to care about coin at all. Lucius is a political creature, so he needs ways to bribe idiots, but a wizard that keeps their newt skills up to scratch is pretty much carrying around a cornucopia machine in their pocket. Sure, you could spend a bunch of effort and rob a bank, then use that gold to have a house built.. Or, you know, save yourself the hassle and raise a cute little tower from the bones of the earth/bend space and live like a king in a post office box.. ect.