Perhaps you don’t quite grasp history before satellites, automobiles or even rail, but colonizing a massive amount of land that you don’t own is difficult for a rich nation, much less the 19th century US. Colonialism is hard work. Again, if you know history, you would see the numerous examples of what struggles it leads to not just for the colonized, but also for the colonizer.

Further, if you really did have a working knowledge of US history, you would probably not have brought up conquering mexico, as there was a two year long war that ended with the US seizing over 1/4 of what is currently the continental US.