Why Isn't Functional Programming the Norm?

Because it solves nothing standard C++ style OO doesn't, and misses out on some of the good stuff that OO does.

Passing a map around in FP, and modifying it as it moves is virtually identical to adjusting a member_variable. Now however you can't protect the changes by having a setter.

After years in c and then c++ then rails and now elixir. I honestly don't see that there is a benefit to functional programming. My unit tests are the same, and the code is just as buggy regardless of approach.

It is never the OO vs FP that make the difference.