Why were couch gags frequently reused?

I've always sort of wondered why couch gags would be reused on The Simpsons. Sometimes, like with the extra-long circus one, you can at least attribute it to them needing to stretch the runtime of an episode slightly. But why otherwise? Like, they could turn out a new 23-minute episode of animation every week, but an additional 10 seconds of new couch gag was breaking the bank?