I think it's just cultural: American men are not expected to be discreet or embarrassed in "practical" all-male environments such as locker rooms and public toilets. A loud belch or other powerful, resounding emanation signals manliness... and it was funny when you were 7, it's still a bit funny now.
"off-screen sounds" as they're called in entertainment, meaning sounds that have no visual cue to explain their existence, ignite a fleet startle response from ear to brain. The science behind the funny and the awkwardness of farting or belching. When expressed loudly, might not even be that far from the roar of a large carnivore.
