I thought so too until I actually used one. The experience is worse than I ever imagined possible (sure there is room for improvement). Additionally the two you mentioned only support ERC20 tokens.
There are some dollar-pegged ERC20 tokens like Tether (backed by dollars) and DAI (a stable coin) that could allow what is effectively fiat-value <-> token decentralized exchange, but to acquire actual fiat, that can be used in the rest of the financial system, requires interaction with a centralized exchange that has a banking relationship.