Because they can.

Most FX brokers do not "exchange money", that is in the sense that you can withdraw the quote currency. Because of this, they don't have to deal with money laundering.

FX brokers and MtGox also have different business models. Brokers usually make money off their spreads, slightly adjusting their quotes in their favor. Traders on MtGox trade directly with other users, not against the company. This attracts different crowds.

Fiat is also not as volatile as Bitcoin. A .35% fee on Forex would completely devastate profits. Meanwhile, Bitcoin rises and falls 10% on a good weekend.

Bitcoin exchanges would be comparable to stock exchanges like NYSE or NASDAQ, except volume is lower in the tens of magnitudes.