Because that severely shortchanges you, at least if you run light and darkness anywhere close to resembling reality.
For instance, on a moonless night, you can spot a cigarette from quite a distance. And in a lightless cavern system, you can pick up the light from an approaching party of heroes from far away, even when that light isn't direct, but bounces off of several cliff walls.
That is, carrying a torch gives you away, period.
You have always been able to create an all-Darkvision party (four Dwarves, say). The annoying difference now is that you can go Half-Elf, Gnome, Dwarf, Elf and practically replicate the "standard" adventuring party (Human, Halfling, Dwarf, Elf) and STILL gain the not inconsiderable benefit of not having to carry any light.
And just because I've been in this discussion - no, the disadvantage on Perception isn't really a problem: most monsters have very poor Stealth. If anything, getting -5 to your score fixes the big that almost everything is trivially easy to spot!
Besides, in many cases the dungeon provides light for you, for most of your trap-finding needs.
Still, the advantage of not being seen means far less ambushes, which is much more beneficial than percepting in the dark is a drawback. (Especially considering the enemies are in that dark too)