What prevents lower class Wizards from being upper class Wizards? For example, the many childrened Weasleys are looked down on by their "betters". Why can't they conjure a more comfortable standard of living? Certainly, the Malfoys aren't "better" than the Weasleys just because they only have the one child.
It's worth noting, I think, that in the UK, wealth and class are related, but not the quite as strongly as elsewhere (like here in the USA, for example). The Weasleys could probably use magic to get a bit further ahead financially, but they will never achieve the status of the Malfoy family, without serious social climbing (however that's done in that society).
And really - when even the poorest families can use magic to remove the drudgery of everyday life, by enchanting a broom to sweep, or the dishes to wash themselves, who needs to be rich, anyway?
First, and most obviously, money doesn't buy class.
Second, while it's not implicitly stated, I think the conjuring of money is likely one of Gamp's Laws of Elemental Transfiguration, meaning it is impossible to conjure money. We know that food cannot be conjured from nothing; I would think this would be doubly true for money. For example, in Deathly Hallows, when the trio is in the Lestranges' vault looking for the Hufflepuff cup, they trigger the Geminio Curse, which causes each item they touch to multiply; Griphook quickly mentions that the copies are worthless -- it's only the original that's valuable.
Third, take a look at the Malfoys. J.K. Rowling shares on Pottermore that the Malfoy family has become one of the wealthiest wizarding families in Britain through ingratiating themselves where the wealth and riches are, including non-magic circles (i.e. Muggles). JKR says that it has been rumored for years (though never proven) that the Malfoys dabbled successfully in Muggle currency and assets, and over hundreds of years have managed to add to their lands in Wiltshire by annexing their Muggle neighbors' properties. They associated with the upper echelons of Muggle society and royalty, adding to their wealth with treasure and art. Most importantly, the Malfoys weren't above using magic to further their own gains through coercion and influence.
Compare the above to the Weasleys. Would the Weasleys be happy working the room and cultivating shallow friendships? Would they put their neighbors out in order to increase their ownership of more property? Would they essentially sell their child to Voldemort just so they could scrabble at the chance to remain in the good graces of a mad man? I personally don't read the Weasleys in that light. What they value most isn't tangible in the way that money is.
Finally, as canon says, magic just cannot fix everything.
I think that it is a combination of 3 factors, 2 of which were mentioned elsewhere on this post:
The concept of class is not entirely synchronized with the concept of wealth in Europe, including the UK. One of the best examples of this would be Goblins who aren't even considered equal to wizards yet probably are richer than most wizards.
The second is that the concept of class is somewhat correlated to "pure-blood wizard" and lineage, but not always so.
As an example, Gaunts (from swamp inbred bankrupt hick Marvolo, to Voldemort) consider their own lineage to be "noble" and "high class" (witness Marvolo talking down to a Ministry official) due to their lineage and blood status; whereas I have a hunch - not supported by canon - that Malfoys and Lestranges would turn their noses way up at a preposterous suggestion that Marvolo Gaunt was somehow of their class.
Similarly, one of the oldest known wizarding lineages is that of Ollievanders (who opened up shop in Britain ~1000 years ago - way longer than canonically written history of Malfoys). But they are in a crass trade craftsmen class, so aren't considered upper crust like Malfoys despite old lineage.
Third one is the one mentioned elsewhere on SFF - there doesn't seem to be a way to magick yourself wealth OR material posessions from thin air for Wizards.
There is very thin canon support as far as why (see my Q here: Why are there "dilapidated" buildings in Harry Potter? ) but the canon does support the fact that fairly powerful wizards for example can't just fix themselves good cloths (Lupin) when they can't afford to buy them or a good house (Weasleys).
In summary, what prevents "lower" class wizards from becoming "upper" class is hard to answer because:
- the definition of upper/lower class is not very firmly fixed and depends on one's point of view even in-universe; but it seems to be at least somewhat correllated with BOTH wealth AND blood status;
- You can not buy yourself an upper class standing in Europe/UK the way it's possible in more egalitarian/meritocratic societies like USA even if lower class wizards could become richer;
- Canon supports that becoming richer isn't a trivial thing for a poor wizard, even with decent magical talent.