It's cheaper than not doing so.

Same as on a small scale. It's been proven time and again that helping people is way more cost effective than policing them. It's common in modern cities with a high quality of living to see things like free public transportation because cities realized they were paying more for police to try to catch or prevent fare dodgers than they were losing lost fares.

No one wants to live in a poor neighborhood. On a small scale you can move to a "better" place and forget those poor people exist. That cannot happen on a global scale (yet).

Solving the economic problems of central America would be way more cost effective than any dumb wall.