Poker players outplay their opponents and you can't outplay anyone if you open too many tables.You can't do it because you can't watch them play. Your attention is always where's your action and you can't see your opponents play with each other.
Multi-tabling isn't usually about outplaying opponents, although the better MTers can still do that to some degree. The whole concept behind MT'ing is exploiting tiny edges in volume. Your bb/100 decreases since you're playing fewer hands, but your winrate increases because you're getting more playable hands.
Also, virtually all good MTers use a HUD so they do have information on their opponents, especially the regs, at a glance.
There are way too many insanely good MTers making a killing at 16-20 tabling or more to suggest that it's not poker (or not profitable). It just takes a lot of practice and not everyone can adapt to it I guess.
When I first tried, it took all of my concentration to play 2 tables. The biggest hurdle was adjusting my game so as not to try and outplay people as often. Get in when I think I'm ahead, or get out -- don't try to get tricky. I think this is where most new MTers go wrong, I know I did. But once that settled in, I added 2 more, and then 2 more. Now I comfortably play 6-8 tables. Although I'm on a terrible downswing with ring lately, but that's more due to tilt and self-control probably than anything else, plus I get these spells of high-variance play style that I have a hard time shaking sometimes. Like when I've settled into a groove of running over a passive table, then not making the adjustment when TAG regs sit down. So I've backed off a bit on ring and am 1-2 tabling MTTs/SnGs for awhile -- the last time this happened, I won a major MTT and cashed big in several smaller ones, so it boosted both my BR and my confidence. Here's hoping for a repeat...