American immigration question?

i was just wondering why america doesnt offer to make mexico a new state as they did with hawaii? wouldnt that save billions of dollars a year on immigration enforcement? i mean they are part of the same continent already and they work just as hard if not harder than alot of americans, so wouldnt that seem to be a better solution to immigration enforcement? what would the downfall be of adding mexico as a new american state? logical & respectful answers only please, thanks.....