The money we send a lot of these places are to help people or buy off officials. Either way, places that take our money are more likely to do the U.S. bidding.
Mexico needs to be propped up right now. Hell they have came pretty close to being a failed government a few times in recent years. They don't have the income to buy a wall, and if they were to fail as a government the amount of illegal immigrants will be way worse.
You ain't wrong. But they're already an enemy on many fronts. Corrupt, bankrupt, and unwilling to stem the flow of drugs and criminals across our border.
$$$ speak more loudly than diplomacy. Who's gonna prop them up if we reduce our world-welfare footprint?