A lot of the actual, serious ones that knew what they were doing got caught. Some went to lulsec to be jerks with no agenda and were caught by the Feds. All that was left were script kiddies that downloaded the Low Orbit Ion Cannon and used scripts they find online. Then they left or were overtaken by alt right idiots.
The original Anonymous are in their 30s and 40s by now. Everyone ages out.
90s script kiddie here - a bunch of the shit you can do as a minor with low/no consequences becomes SERIOUS FUCKING BUSINESS as an adult with assets. It's just not worth the risk to keep dicking around with things that might land you in prison or cost you everything you have.
I think the serious ones that didn’t get caught are now working in red team penetration testing, which is an industry that’s been growing exponentially since the years Anonymous did a lot of their big stuff