He left after season three, the first movie was meant to be the end.

And even if someone doesn’t care about that sentimental stuff, it’s also most likely a terrible movie. Spongebob as a whole isn’t worth watching once Hillenburg left after the first movie since he intended that to be the end of the series. Post-Hillenburg Spongebob is little better than YouTube kids videos made by content farms, sometimes no better.