Power
The power of a sculptor is a tangible, hands-on kind of power (although we strongly advise against touching anything at the museum). You're probably never going to win an election, fight a war, or lead a revolution, but if you have the right skills and reputation you'll be the one our government calls on to commemorate those things. Whether it be a democracy or dictatorship, monumental sculptures are one of the prime displays of choice for the powerful.
Just look at Washington, D.C. From the giant Abe Lincoln sitting like a stone goliath on the National Mall, to the busts of every president we've ever had lining the halls of the White House, to the nineteen-foot Statue of Freedom that you didn't know was on top of the Capitol Building until just now, there's as much sculpture in D.C. as there are loud voices and shady dealings.
All of these things exist to remind us that our government is for the people, will exist much longer than any of the people, and that we really, really enjoy copying the ancient Greeks, Romans, and Egyptians.