Time is a factor. Taking land is what was done in the past. US wanting to do it now is as if Mexico decided to revive slavery and threatend to capture Afroamericans in USA. It's a touchy subject and I think most of US wouldn't be exactly on board with this idea.

Especially since putin shows us exactly what happens if you try.

> Mexico decided to revive slavery and threatend to capture Afroamericans in USA

It's a bad example because the power balance doesn't make sense. I think you will have a hard time coming up with any example where USA would be on the receiving end of something like this.