Diplomatic actions have consequences. You write we are powerful we can force Mexico to build a wall. That is simplistic thinking.. what happens if Mexico legalizes drugs? or signs an alliance with China or Putin? or devolves into Syria and now you have 120 million people trying to cross the border instead of a few hundred thousand? No army division can hold back that many people.
Those are the many reasons why a play yard bully taunt wouldn't work. Anybody with an inch of political experience could see this. But that is not who Trump is talking to, he's aiming for the politically naifs who go "hell yeah", because the taunts make them feel good.