Tablets are way more expensive, don't control as well, don't have high enough graphics fidelity, and don't offer the same gameplay experience. A tablet can be a fine companion device, but it won't take over as your main game device any time soon.
You can argue that they have some nice looking games, but most of them are a controlled linear experience. There are tons of clones, and the popular titles get lost in the sea of a constant stream of apps coming out.
I play games casually (when I just want a distraction) and hard core (when I want a couch experience that's going to last a few hours at least) and I'm happy with my 3DS and PS3. I've got a phone but only a couple of games on it, Dodonpachi and Deathsmiles, probably the only games which control better on a phone than on console. I don't see myself getting deeper into mobile games.