Not arguing they aren't useful. Just that no backend dev is ever jealous of what's happening with front end frameworks.

Another interesting question : are frontend developers jealous of what the backend developers are doing, or are they not?

I sure was when I had to do front end work. Finally got out of anything front-end for good and it's probably been the single most pleasant change in my career ever. I didn't start out doing front end work though, so I could see while I was doing it how ridiculous it was compared to almost any other domain in software dev and only getting worse. A good portion of front end devs I meet have not done anything else so they don't have a point of reference.

Just not always a good reason. You're not breaking ground here by having stumbled upon the concept of causality.