Maybe it's the pressure.
One of the big reasons to do a side-project or a solo album is to get the world off your back. Billie Joe, or something called The Longshot doesn't have every media outlet and most of the world waiting to judge it into oblivion.
So there's freedom in doing those, where Green Day is a business. A business with many customers, a lot of whom are overly critical bastards, haha. With that situation, it's hard or near impossible to go too far away from the "Green Day Sound", no matter how the band feels about it.
Maybe Billie's fired up about a wild new idea.....but no. Those buttholes over on GDA don't like that sort of thing.
He's written the greatest jazz improv of all time.....but 99% of fans will bail. Can't do that either.
I'ma stop right there, because Green Day have never put out a bad album. Not even bad song, in my opinion. Haushinka was pushing it, but even that turd is okay to my ears. Lol