Can’t they? Maybe you haven’t asked enough people. Butts are funny for several reasons. They’re a “private” part in many cultures, so you don’t see them all that often. And when you do see them, it’s often as the result of an intrusion or impropriety when the owner of the butt is vulnerable, which can be especially funny if said butt-owner is not generally the kind of person who seems vulnerable in day-to-day life. If you see a baby’s bare butt, it’s just sort of soft and fluffy and cute, more or less like the rest of the baby. If you see the butt of a youngish adult, you might find the butt to be somewhat sexy or titillating. But if you chance to see the exposed rear end of a king or president or pope or other dignitary or potentate, there’s a moment of cognitive dissonance. Even though we didn’t see his buttcheeks, I suspect more than a few Game of Thrones viewers snickered when Tyrion found Tywin seated on the can. Those people, all-powerful, mighty, influential, the world in their grip, still have to drop trou and squeeze out some buttsnakes every day or so. And when doing so, they’re usually not concentrating on exerting their political power. Hell, they might be constipated. Or can you imagine Aragorn riling up his troops at the Morannon while struggling with a bad case of gas:
“Sons of Gondor, of Rohan, my (frrrt!) brothers! I see in your eyes the same (fzzztt!) fear that would take the (f)heart of me. A day may come when the courage of men (farrrrtz) fails, when we forsake our (ffffrrrttt!) friends, and break all bonds of (frt! frt! frt!) fellowship; but it is not this day! An hour of wolves, and shattered shields, when the Age of Men comes crashing down; but it is not this day! This day we (FART!) fight! By all that you hold dear on this good earth, I bid you stand (oh, Elendil’s Entrails, I soiled my armor!) Men of the West!”
And the products of the butt: shit & farts. Undignified goofy sounds, and foul odors. The potential source of much mirthful mischief. And butts are often chubby, usually soft and relatively pale (being a place where that sun don’t shine), and the buttcrack is frequently hairy, often be-dingleberried, and somehow doesn’t look like the end of the hero that one would normally immortalize in a bronze bust. Why?
'Cause it’s funny.
