At the moment, they're significantly more expensive and less compatible. They're also, by virtue of design, far easier to steal / lose and we've had quite enough (UK) stories about laptops being left on trains by mistake or stolen from homes and critical data being leaked as a consequence. Also, I can't see many people wanting a 10" desktop monitor or a 17", let alone 22", tablet.

I don't dispute it could happen, but I'd be surprised.