It's like spending ten years learning to speak Russian and then criticizing anyone who says that learning Russian is difficult.
Puzzling out scalaz code is difficult and requires an enormous investment in hours and practice, investment that a lot of people prefer to put into different learnings.
It's incredibly multipurpose, more so than even Spring or Guava or LINQ, and these are things that developers regularly have to invest serious time in.
The argument is just that FP libraries (like Scalaz) have a bigger payoff in the investment.
At Verizon Labs were have 20+ microservices that I have touched/looked at. Some use Akka, some use Play, some use Jetty, some use Http4s but everyone makes use of Scalaz somehow.
It depends on the people, not everybody has the inclination to dive so deep into hard core FP and they will be more productive using a different approach.
Don't make the mistake of thinking you've found the only software silver bullet that exists and that people who don't use it "don't get it", which is another attitude I've seen a lot of hardcore FP advocates embrace.