Since wealth by level is universal for all classes, they are all equally dependent on gear. The dependence is more quantifiable for martial classes, but it is similar for all. For example, suggested WBL for a 10th level character is 62,000gp. Take all that away, and every class is going to be much less powerful.
Ok, a bit more context. No crafting, no picking your choice of item in a shop, no having your existing weapon/armor/wondrous item enhanced by a handy mage for a few 1000's gold.
You get whatever loot you find as treasure, that is randomly generated, but with control to prevent finding items that are too powerful.
Maybe buy the occasional potion or scroll. But good luck finding even a wand of CLW for sale.
Who's able to do the most when it's impossible to plan what kind of gear you'll have? (Mundane items are ok tough, the limit is on magical treasure and rare goods).