Yes, but if it were just that, you would except a few low-status wizard to suddenly become very rich through muggle-side tricks. Arthur Weasley is probably too Gryffindor to do it himself, but since he has quite a lot of work with wizards doing tricks with "muggle artificats", you could except a few of them to get very rich by fiddling with the muggle world (especially muggle born, at 11 you know about stock markets and lottery) if it were so easy.

My best guess is that it's illegal and the law enforcement is strong enough to not be worth the risk. Like, if you suddenly arrive at Gringotts with gold coming from nowhere, an investigation is done, and if that gold comes from a "muggle source", you're in trouble.