Actually you're wrong, not all mexicans are of spanish decent, some and most of them are of native ancestry.
Besides back then the Spanish invaded Mexico and raped the Indians which produced mestizos but they were later droven out of the country even though it was a Spanish colony. Later Mexico and other Latin American countries were called Spanish America.Today most Mexicans are Indigenous and some have little Spanish blood. I think a lot of mestizos today carry about 1/8 or less European. And there's some Spaniards living in mexico today, they're called Creoles(In Spanish,Criollos, born in America of Spanish parentage), Peninsulares(Spaniard born in Spain and reside to mexico) and gachupin(Spanird born in Spain and living in Mexico), and they consider themselves New mexicans.