It's not poorly guarded, it's just unexpected clientele behavior. A ton of smaller museums pretty much expect you to keep your hands to yourself. Guards are expensive and then you have a limited quantity of visitors you're going to get some dumbasses mixed in the crowd.
The excellent Museum of the City of New York up the street from the Met has whole areas without guards, you're just expected to not knock things down. And at the Met if you were determined you could knock down sculptures before the guards could get to you. It's just that you're not supposed to.