That adds a round of latency, and makes it pretty much impossible to boot off the drive. The blocks aren't in the same order in the Flash as they are presented by its interface, and one of the main jobs of the controller is to re-order them.

It would be an interesting product to have, a raw block API to a Flash device with all the temporary state stored on the host - but a hard one to sell, as it's not differentiated in any way.