But that's my point.. If there is a site where you can sell your BTC for fiat, but not get the fiat out of the site.. would you sell? No. There would be nobody selling coins for fiat, so that's not an option.
I guess if you don't trade much you might think that, however in reality I use to buy and sell fiat all the time on BTC-e yet I never deposited or withdrew any. It is a hedge when BTC is crashing like USDT is on Polo, people love the stuff.