It's because most products in the US are sold in multiple states, including California, and the logistics of ensuring your "known to cause cancer in California"

It's because most products in the US are sold in multiple states, including California, and the logistics of ensuring your "known to cause cancer in California" warnings are only printed on the "California version" of those products just isn't worth the effort. Better to just have one version of your product that can be sold in any state.