Kelso

its just his character in the show. he makes people laugh by acting like that. there is always one like that in a group just like in friends which is joey.

Because it makes people laugh. When sitcom writers find something that makes people laugh, they use it to the point of abuse. Look at Friends -- Joey was never smart, but by the time the series ended he was just a bumbling fool. And on Frasier both Doctors Crane had no common sense by the time that series ended.