[Loyal]

Foresight, et al, doesn't cause one to always hit; that's what Magnet Lock and its kin do. All this does is reset the foe's evasion - One additional double team from the opponent and you're back to where you were before. They will always be one stage of evasion ahead of you, especially if they're faster (which, considering the pokemon likely to learn these moves in the first place, isn't a far stretch).

If you reset their evasion, they can either raise it again, or take a free shot at you because you're too preoccupied making sure you can take a shot at them. If you try to just brute force through the evasion... well, we know how fucking annoying that can be.

It overcentralizes the game, limiting viable strategies to evasion and counter-evasion teams; and that is why it's banned.