I cannot speak for anyone else, but when I was pretty young in my career, my first "real" job was doing ColdFusion. Even at the time (early 2012), ColdFusion was largely dead, and PHP had sucked up most of its potential audience, and I was looking to learn a new platform to future-proof myself and make more money.

I had learned the core Java language years before and I knew Java was "enterprise", so I figured that might be a good spot to start, so I bought a book on J2EE development with GlassFish, downloaded all the appropriate SDKs, and after multiple days of spending nearly every minute I could on it, I got something not quite as good as something I could have whipped together in ColdFusion (or Railo/OpenBlueDragon) at the time.

I thought that maybe I was just stupid, and I suppose the jury's still out on that, but shortly after that experience I found a Django tutorial series on YouTube and went through that, and felt like I was productive almost immediately. I was able to build "real" stuff in a matter of a few hours instead of days, and it didn't feel like it was more chaotic than the stuff I had done with J2EE. I then did a similar experiment with Rails, and had the same effect: me being an idiot is not solely to blame for me not really grokking J2EE. Eventually I learned Node.js and did that for a few years, until I was able to (thankfully) transition out of "web development" entirely.

Personally, while I'm sure the Glassfish server, and maybe J2EE has its merits, I don't think they're worth the headaches I went through to try learning it. Java has plenty of cool frameworks now (e.g. Play, vert.x), but I've never even tried using those with Glassfish.