Why are there any poor wizards? They can so easily get rich with muggle money and then exchange it.
Because of the Office for Prevention of Uunfair Advantage, Ministry for Magic. This office is in charge of ensuring that no magical powers will allow a wizard to use his/her skills to better a Muggle. And couple that with the stringent rules of magic in front of Muggles. Besides the fact, most (pure blood) wizards are too clueless about Muggle life to be able to do anything in it.
For a real in-depth analysis of this, read Harry Potter and the Methods of Rationality.