The assumption that Cats Effect / FS2 / ZIO is easier than Akka / Pekko is questionable. There's the assumption that the Akka ecosystem requires the entire system to be built using the various features the ecosystem offers, when in reality it easily scales down to simple microservices just as well, if that's the only option or the overall preference.
One reason to reach for the pure functional libraries is a desire to avoid some of the many footguns Akka / Pekko has. For a long time, using Akka meant not being able to rely on the Scala type system for checking messages sent between actors, and for the people attracted to Scala due to the advanced type system, this was a deal-breaking drawback.
All in all, the FP libraries and the actor system libraries are almost direct opposite approaches to problem solving.
Well, Akka/Pekko is typed now. There is a learning curve to Akka, but it took me 3-4 weeks to get my system working. I can't imagine doing it with pure FP, it would break my brains.
As someone said - these are very cool but niche capabilities that companies do not talk about. It's almost like a secret weapon in the sea of over-engineering and hype.
One reason to reach for the pure functional libraries is a desire to avoid some of the many footguns Akka / Pekko has. For a long time, using Akka meant not being able to rely on the Scala type system for checking messages sent between actors, and for the people attracted to Scala due to the advanced type system, this was a deal-breaking drawback.
All in all, the FP libraries and the actor system libraries are almost direct opposite approaches to problem solving.