Keep in mind there are two types of monetary wealth in the HP world.
Muggle wealth is the money that we are all familiar with, and can only be used to purchase goods and services from Muggles.
Wizarding money is used to purchase goods and services from Wizard society.
For the most part, there's very little that members of Wizard society want or need from Muggles. Property, housing, and food really seem to be about the extent of it for the vast majority of Wizard families. Most families already have established properties and houses, and using magic would, in most cases, make expanding or modifying houses much easier. Which leaves food, for those families who don't already grow or raise their own. However, food can most likely also be obtained from other members of Wizard society (in a market as crowded as Diagon Alley there's almost certainly some grocers, and as further evidence many wizards are so unused to Muggle society that they are incapable of fitting in for even short trips, which implies that they are not making regular trips to the supermarket for food). Add to this mix the convenience of traveling to Wizardly destinations via Floo network, and it seems that the need to purchase from Muggles is a relatively rare occurrence.
Even if a Wizard or Witch did want something from the Muggles, and conjured up gold, or jewels, they'd have to find some way to convert them into Muggle currency. Selling a small fortune in valuable merchandise is not a quick and easy process if you attempt to do it legally, especially if you are shabbily or even oddly dressed, and seem completely unfamiliar with the relative value of currency.
In short, it is no doubt possible for a Wizard to get very rich in Muggle currency, but it is not necessarily easy, convenient, or altogether useful to do so.
As for Wizarding money, it is almost certain that it cannot simply be summoned or transmuted. The whole point of currency is to act as a convenient marker for goods or services that have value and are worth exchanging for goods or services of similar value. In order for currency to be a convenient marker, everyone using it must agree to its relative value, and agree to keep that relative value stable. I think it can be assumed that there are checks and balances in place to insure that magical forgery of the currency is difficult, if not impossible, and that anyone caught doing so would be severely disciplined.
If conjuring gold, or transmuting glass into diamonds, or some other means of conjuring "wealth" would be possible, then those materials simply would not be valuable to Wizards. Why would one wizard buy or trade for diamonds that another wizard conjured or transformed, when they could almost certainly do the same themselves?