Why do cartoons and other American shows never change openings?

[CommonMan]

Oh god, I LOOOOOOVED THE GUMMI BEARS! I still eat them to this day.

Tonight, Tonight, Tonight from Bleach is teh best opening music in the history of opening music. Except that it was running during the terrible "Bounto" filler arch. But it can't compete with the battle theme: "Got to be the center. . .OF ATTRACTION!" and Jazz Hands!