The border from the Pacific to the Gulf of Mexico is much longer than the Isthmus of Tehauntepec (spelling?) at Mexico's southern tip. Check it out on a map of Mexico.
The land border is shorter but the seacoast is waayy
longer.
Absorbing Mexico would gain us little more than another
New Mexico with crooked politicos and mostly dirtpoor
ill-educated peasants.
Better to absorb say, Taiwan or Singapore.