It has nothing to do with a lack of ambition.
You know, maybe this is a generational problem. Because when I picture why my PCs go adventuring, it is rarely because they want to show the world how great and mighty they are. It is because Gnolls are attacking the nearby village and killing people and need to be stopped. Or some crazy cult is trying to summon Orcus to the mortal plane, leading to the death of everyone and needs to be stopped. Or Mind Flayers have infiltrated society and are turning the people into mind slaves and need to be stopped. Or Giants are gathering an army to pillage and raid and kill people and need to be stopped. Or an Archdevil is trying to find an artifact of power to draw a city into the Nine Hells, dooming thousands of souls to a fate worse than death and needs to be stopped.
You might have noticed a pattern.
And, in response to these threats, the races of Men, Elves and Dwarves have banded together to fight to keep people alive and free.
Meanwhile, Halflings have kicked their feet up by the pond, fishing, and relying on humans to keep them safe. And their most iconic class is the thief, sneaking and stealing from the fellows in a bizarre manner that seems completely at odds with their supposedly kind and generous nature.
And maybe, if you drag them kicking and screaming, they will actually stand up next to the other races to try and deal with the world's problems, but for the most part you guys want them to be isolated in their little perfect communities that are never under threat, because someone needs to be unimportant common people in a world full of common people just trying to get by.