I noticed this too, and I have a couple theories about it. Since the actors only contribute a voice performance, including a musical number might be easier to d

I noticed this too, and I have a couple theories about it. Since the actors only contribute a voice performance, including a musical number might be easier to do in animation than in live-action. There's no on-screen talent that has to learn choreography or whatever, no difference between drawing the show's usual sets and whatever outlandish one the musical requires, and so musical numbers already fit the production of animated tv. The other thing I notice is that a musical number allows a single premise to be mined for multiple jokes and the dialog passes more slowly as its sung. I've wondered if writers sometimes run out of ideas and think it would be easier to write a couple verses of parody lyrics rather than new jokes.