While I agree she's overly reckless for her age, few historical royals were raised in a comparable situation.
It appears there's only about 3 noble families in Mewni, because there were only 3 castles in the Kingdom when Star took the Diaz's there for their anniversary. This means there's few contenders for the throne, as in history up until the late 18th century, Kings were replaced by Nobles, not democracies.
People like Louis XIV and Ivan the Terrible grew up understanding that the only thing separating them and the Nobility was tradition. Most nobles aren't willing to risk the precedent that "right of conquest" establishes, relying on hereditary rights to maintain their dynasties. If the King is particularly weak though, most nobles will fall in line behind one of the few ambitious claimants who are stronger. Those men understood this, and despite being children when they ascended, they understood the need to project strength to separate the Crown from the nobility.
The Butterfly's though? It seems the Queen and Crown Princess are the only two capable of magic in the entire planet. That may be because no one else has a wand, but they also seem innately attuned to magic (the whole dip down and butterfly form). Moon goes head to head with Toffee who has a wand while she is relying on innate power and we've seen that the wand is capable of immense power. There's no threat to the Mewnian Kingdom that Star would've been raised to consider credible. Throw in the absentee parents (Star was raised by the guards by her own admission) and it explains a lot about why Star isn't prepared to be a Queen.