Why not liberate Mexico?

Since so many mexicans are aching to be American and there are vast natural resources especially Oil. And since they have no military to speak of which is made evident by the amount of Illegal drugs flowing through the country. And since they are sending so many men here to fight as mercenaries with citizenship for pay. Why not annex Mexico and utilize they're cheap labor, take their oil, and conscript their men into military service all the while saving our selves alot of trouble by reducing the length of our southern border to the length of Mexocas southern border.