The thing is, there are basically three kinds of psychic powers:
First, there are ones with no cooldown. Mostly, this means Light Manipulation. You can rely on this to kill weaker enemies.
Second, there are the ones that reduce their cooldown as they go up in level, like Teleport, either of the two forcefield powers, Clairvoyance, and Mass Mind. Notice that these tend to be defensive powers, or ones that can be used to keep away from opponents. (This is, I think, necessary for the game to avoid having max Willpower be totally essential to survival as a high-level esper.)
The third group are ones with cooldowns that don't decrease -- Sunder Mind, the AOE attacks, etc. And yes, sure, high willpower helps with these... but the thing is, when you have Teleport on a 4-turn cooldown and Clairvoyance + the forcefields at very high levels, does it actually matter if it takes you a bit long to recharge Sunder Mind? Usually, when I use these powers, it's to finish off enemies I've already dealt with using the powers in group 2. I don't care about waiting a bit longer to use Sunder Mind, because my other powers are the ones that are actually keeping me alive.
On the other hand, what is a threat to a high-level esper like this? Stuff that takes you by surprise. Situations where you screw up. High HP helps you a lot more against that than being able to Sunder Mind 15% faster -- if I'm unable to hold the enemy down with forcefields or keep my distance with teleport, 15% faster Sunder Mind speed isn't going to save me.
I mean, that's just my theory on it, and I'm sure there are other builds (maxing Willpower instead of Ego definitely lets you do some cool things; clearly there's a difference between being able to teleport every 5 turns and being able to teleport every single turn. And Ego eventually has diminishing returns at ridiculously high levels, while Willpower eventually becomes worth more and more -- but those are at levels you won't actually reach in-game.)