Well that's actually quite simple; because most Americans were against it. There were always a lot of protests against any of those wars, the public didn't find those wars worthy the cost of any American lives and in a true democracy politicians actually pay mind to the public opinion for the simple reason that they want to gain votes and not lose them. It's different in a country that's ran as a dictatorship (no matter what kind of governmental form the country in case "officially" utilizes) because in that case you can just easily brainwash most of the population and silence the others - but that's not the case in any Western democracy, as flawed as they may seem. Aside from that, wiping another country from the face of the earth will most probably turn your country into a pariah state of which the cost is likely far greater than the cost of "losing face" in a retreat.

Honestly, I always find those kind of "Lololol but the USA lost the war in Afghanistan and Vietnam rofl so they can't be that powerful lulz" comments so embarrassing in how completely oblivious they are. So many people don't seem to have the faintest clue about what the American military actually has at its disposal. Sure a country like Russia or China are capable of nuking the shit out of another country, but the Americans wouldn't even have to use nukes to turn another country into one single gigantic crater. You're free to believe whatever you want, but I know for a fact that in this day and age anyone should still think twice about trying to truly piss off the American military. Because if the vast majority of the American public would actually care and if they would actually feel sure about another country (or countries) being an existential threat to their existence they would be entirely capable of transforming the country in question into nothing but a speck of dust.