Spot ForEx

Why is it that most retail discount brokerages (E*Trade, etc.) don't offer access to the spot foreign exchange market? Some of them allow you to trade forex futures but that's a whole different ball of wax. I don't get why spot forex access isn't more widespread.

It's not that you can't get a brokerage account that provides access to that market - you absolutely can - it's just not ubiquitous like equity or commodity trading accounts are. I'm curious why that is.