Actually, it's a very decent question, and nobody's ever asked before. The reason is that one "free" super-power is what super-characters get in exchange for not being human.
Look at the bonuses given to the human origin. Three extra Hero Points at 1st level, a double gain of level-based Hero Points, two bonus feats at 1st level, further bonus feats at 5th, 10th, and 15th. Plus lots of extra skill points.
Look at, say, the mutant. 1 Hero Point at first level. No other major advantages... except full access to super-powers.
A mutant can take just one super-power at no XP Tithe cost. If he takes no further powers, he advances as fast as a human does and has a special power largely equal to all the human advantages. Although, realistically speaking, all the extra feats, skill points, and Hero Points are *slightly better* than a single super-power, but perhaps not a spectacular. Flight, for example, even at just one level, is so useful it would take days to catalog all of its potential applications.
Does that make sense?
Cheers,
SL
