It's just that I can't believe that the average wizard thinks of muggles as persons (or humans) that can be stolen from.

I don't know. They have a fairly insular view of the world, but putting them on a par with animals seems to be a fringe notion, albeit one popular with those in power (ie purebloods.)

The reason bribing people with money works in the first place, is that most people don't have as much money as they would like. If wizards didn't really need money, as you suggest, then they wouldn't care about it and couldn't be bribed.

Magical goods can be sold for money, and therefore are. You want a new broomstick? Pay up, bucko.

(Also, food is hard to create magically, according to canon, although God only knows how that works.)