Unsurprisingly, the answer is money.

Let me save you a click and having to see all those ads:

“If I had infinite money, I would put a couch gag in every episode that would just pop up on streaming, and you’d be surprised by it, but we have to put our money into what’s in the main product first.”

The Simpsons is currently in Season 37, and has been renewed through Season 40.

I'm not even a fan, but this seems like bad stewardship. It really feels like they've stopped trying.

So I looked it up and they definitely are doing less couch gags this past season, but it's been that way for the last three seasons. Additionally they've been doing quick intros also without couch gags for the last 10+ years.

I do think it's worth the effort and it's one of the things that make The Simpsons unique. And they're not 100% gone. However it hasn't really hurt my enjoyment, so I guess it's fine, but I would like more of them.