Why do cartoons and other American shows never change openings?

[Mr Khan]

Damn you, dgc, you beat me to it :P

 

The Simpsons changed opening sequences, though it took them 20 years to do it. Venture Bros axed its opening in the 4th season. A more memorable one might be The Flintstones, which originally had a very generic opening

Before they got the extraordinarily memorable

Courage the Cowardly Dog got updated images (new villains rosters with the new seasons). Rocko's Modern Life had two openings, both with the same theme song, but two different variations of it, iirc.