Because there are a bunch of nerve endings all squished together right there.
Probably not very but this is a guess. Any excess sensitivy would have been mitigated by contact with clothing and the individual would compensate naturally in any case.
2) If you have an outie does it feel funny to touch it?
Just a sample of one, but…My 2 year old absolutely will not wear her pants with the waistband around her waist, but must push them down under her belly. She is the only one of my 4 kids to do this and the only one with an outie. If I pull her pants up she says, “no! boo-boo!” so apparently it is uncomfortable.
We went to the beach last week, and there was a kid…about 10 years old…running around and it looked like he had a small hot dog hanging out where his belly button should be. WTF was that?