It's also laziness and controversy. Unit testing is something aggressively pushed in many of these places, but then they continue to litter side effects, long flows, loads of complex objects, void methods, and more which inherently make it more difficult to test. FP guidelines help a lot of these cases, and you don't need to understand monoids, monads or any of that to grasp the concept of simple functions with clear output.
Meanwhile, FP is kind of a pain to connect things in, for most people.
Before learning FP I used to ask myself: "Assuming that by tomorrow my memory gets erased and my IQ diminishes by 50 points, will I understand this code?".
That's why I try to juggle the least amount of variables as possible at the same time, while trying to have my code to read like a cooking recipe or a very simple literature. From that, FP feels almost natural.
Even more so, I think that most software bugs and spaghetti code is made by people that were too confident on themselves. They thought they could handle a lot of random unrelated variables floating around all the time, while having their functions do a lot of random algorithms at the same time.
And yes, they could handle it. The first time. But, in the long run, most people forget, or they leave and another person ends up having to maintain it, and that super brief-and-clever code became an unmaintainable mess.
I'm trying to push unit testing, as a means to get rid of some of those negatives you pointed out. But it's not a panacea. And obviously, I don't want to test for the sake of it either.
Meanwhile, FP is kind of a pain to connect things in, for most people.