That is not the only reason. For example, compare a 10th-level Psion against a 10th-level Warmage. Without ability modifier, a Psion has only 88 pp. That is only enough to shoot eight 10d6 area attacks. On the other hand, a Warmage has 6 3rd-level slots, 5 4th-level slots, & 3 5th-level slots. And he can use all of those 14 slots to shoot some 10d6 attack spells. There are damage caps for each spells (Like 10d maximum for Fireball), but in overall, a warmage can use all of his highest to 1-2 levels lower slots for causing "level d6" damages. And there are spells like Everd's Black Tentacles, for which almost only the caster level matters. So in regard to the number of "top tier" attacks per day, Warmages and Sorcerers tend to trample Psions.