There's a sort of logic to having a weak military in Mexico. Their only land neighbours are the USA, Guatemala and Belize. Neither south neighbour is a threat, and despite the USA being really inefficient there's no way Mexico could fight toe-to-toe with them if they were directly invaded.

They also don't have any strategic overseas rival like an imperialist power would.

So not only can you save lots of budget in the military, but it also reduces the harm that a well-funded and organised military can be to the "internal enemy", which is usually poor, indigenous, black or otherwise marginalised people. (Usually with the cover of "combatting drug trafficking")

Bribing the military is also part of the CIA modus operandi for interfering in Latin America. Just look at the current investigation on the Bolsonaro coup attempt and how many of the relevant generals took part in the MINUSTAH.

But I also don't think the USA will ever be able to directly attack Mexico without gigantic repercussions to themselves. They're better off just doing another Allende.