Evolve a L30 Abra 9a15d13s?

Attack stat is beneficial on every Pokemon but doesn't mean each Pokemon benefits equally from it.

Alakazam already has high attack that the relative difference from IVs is lower than on Pokemon with lower attack.

Your point about someone powering up a Mewtwo even if it had low attack IV is just proving the same idea - Pokemon that already have high base stats but low IVs are not significantly worse than the same Pokemon with 15 in that IV.

Whereas something like a Chansey 15 attack is much more important because it's base attack stat is only 60, 15 attack IV is a 25% increase in it's attack while 15 attack IV on an Alakazam is only a 5.5% increase.

Also in the case of Alakazam against a Machamp raid which is presumably what most people want one for, Psycho Cut doesn't increase in damage until at least level 34 with 14 attack IV, which is unnecessary to power Alakazam that high since Machamp is an easy boss.

For Confusion, it's a difference between 27 and 28 damage per use of the move, which is not as significant because Confusion is a slower move and also already does high amounts of damage

Going from 27 to 28 damage is only a 3.7% increase, while going from 7 to 8 is a 14.3% increase in damage, but is unnecessary. If your Alakazam is at such a high level and your other 5 Pokemon are also decent then soloing Machamp should be quite easy even against Dynamic Punch - I soloed one with BP/DP with 6 level 30 Pokemon.