There's enough gold per value to make a successful thief the richest wizard in the world. And there have been big gold vaults for many centuries, with royal treasuries preceding them. And yet there hasn't been even one case in Muggle history of all the gold disappearing from a vault without explanation.

Funnily enough, it occurred to me that I have no idea if any such cases* have* occurred. I still don't, because all the top Google hits seem to be conspiracy theories or something about how various gold reserves are totally empty. (Maybe a coverup would actually be possible with magic? Hmm ... Fort Knox hasn't been audited in 60 years, supposedly, so who knows?)

But honestly, who cares if you're the "richest wizard in the world" if you're in jail and your "wealth" is an unspendable stolen asset (remember, raw gold needs to be converted into coins)?

A thief could still succeed once.

Yeah, I just meant that muggles wouldn't actually lose all the gold. It's a moot point, since clearly this hasn't happened either way.