Perhaps that was the point
The point being the movie is ridiculous and written by an idiot? Is that the point? But if you liked it fine. Not my cup of tea.
The natural division in the USA would be the South + the mid-west forming a union agains the rest of the USA. Maybe electing a congress and placing their capital in Dallas. That would be a REAListic civil war movie plot.
PS: CA LOVES the federal govt.
In fiction, it's the writer's responsibility to give reasons and explanations for things that may seem absurd.
The fact that many people ask about the California-Texas alliance goes to show it's confusing to people. A good writer could have added on sentence to fill in the blanks. It's a big plot hole for many people.