There are a few reasons. First of all, all stats are not equal. Edge, for example, is really powerful, and gets better each point you put into it. A human with 6 or 7 edge is very strong on that merit alone.
Second is role play considerations. A Troll will have problems walking through doorways, and with racism. Not something that should be matter for points, but there it is.
Honestly speaking, the character creation system for SR4 is terrible, just awful, older editions had metatypes actually have meaningful costs.