Seriously. Nobody is willing to pay the necessary premium to bring this to market. Indegogo, Kickstarter, etc. provide a way to get funding, but even then you don't see keyboard phones popping up, though everybody and their brother seems excited to build another 3D printer.
What would you pay for a slider, and could you find 1000 people willing to put their money where their mouth is? Android is open(ish) so you don't even have to make your own OS. With a couple thousand people you might be able to get the cost down to $5000 a piece.