Nexus for all!!!

Why does google not release the nexus phone for all carriers at the same time? I understand they may not have the time/resources to release a nexus for every manufacturer (even though we would love that) but why all this carrier exclusive crap. If they did this, the nerds get their pure experience with timely updates and the carriers/manufacturers could skin to their hearts content. This way we can also have a better comparison for nexus vs skin phone sales. 

What's your take?