They can't. They're not coordinated, they're not competent, they aren't remotely meeting recruitment numbers, they literally cannot do this, and some of them know it. They could turn the world to bubbly boiling glass¹, but they can't occupy three medium or larger American cities.

If they try to do LA, Chicago or new York, they're done. All their resources tied up just in that.

Occupying cities at the level they would need to takes a lot of work.

¹if anyone's been doing maintenance on those things; who actually knows how many still work