Because the United States dominates the entire continent by sheer economic and military size, and controlls directly or indirectly almost all of it.
Now if Mexico, Brazil, Argentina or Colombia become strong enough to challenge the United States, that would change.
Not really?
All of the countries are Hispanic and culture is not THAT different between themselves. Why would they fight?
Only reason I can think of are border disputes but those are pretty insignificant. The areas larger in size that are contested we already had or are about to see conflicts involving them: Malvinas Islands and Guyana Esequiba.
Literally every other area contested is pretty irrelevant in size to justify an outright conflict. Maybe the Suriname/Guyana contested area is the only other exception.
And even if you’re saying to fight against the US, what is the beef against them? I can only think about Mexico maybe wanting to regain its territory.
