How come in most school in the USA (at least mine) they teach Spain Spanish instead of Mexico Spanish? Would not Mexico Spanish be an obvious choice to teach?

Texan here. We learned Mexican Spanish (seseo, yeismo, ustedes for everyone, etc) It's been years since I had to use it for my job but IIRC there's a difference in the subjunctive verbs as well.

There are also distinct varieties of Spanish spoken in the US that differ from Mexican Spanish. As a general rule, if a common word has a similar-sounding English cognate (often false cognate) the cognate will be used. truck = troca instead of camión, concrete (as in cement) = concreto instead of hormigón, carpet = carpeta instead of alfombra, to park (a car) = parquear instead of estacionar, and so on. This is from my years working as a bilingual call center agent.