>There's a similar number of people crammed into a similar geographical area on the east coast.

The east coast has unlimited supplies of fresh water. California does not. The east coast megalopolis is also situated on a coastal plain, with a deep hinterland unconstrained by any mountainous geography. California crams a similar amount of people into a tiny sliver of coastal land in between the mountains and the sea, surrounded by some of the driest deserts in the world.