Because nobody calls people from Mexico "Latin". If someone walked up to me and said "Hey I'm Latin" I'd be really confused. You're... a dead language?
> Because nobody calls people from Mexico "Latin".
Yes, they do, “Latin” and “Hispanic” were the dominant terms in English before “Latino” and “Latina” took over and are still current though less common; the main reason Latino/Latina took over is because they respect terminology used in the described community, in a way Latinx (used generically, use as a label for enby members of the ethnicity may be different) does not.