> A web frontend is very resilient, whereas backends are the complete opposite. If you apply the same mindset on both
Even if I accept your premise (which I don't), nobody is suggesting you apply the same mindset to two different things. I consider myself a full stack developer, but if I had to pick one it would be back end as that's where I started, where my strengths lie, and if I never wrote another line of JavaScript in my life I'd be pretty happy overall. But your characterization of full stack devs does not match up to my reality.
I salute your approach to engineering but that's certainly not how employers view it.
I am not saying that frontend engineering is trivial. It clearly has many overlapping and non-overlapping requirements. But they non-overlapping part is large enough to require dedication and specialization.
DevOps and DBAs overlap with backend but it would be weird for a backend engineers to call themselves devops engineers or database administrators because of that. The latter suggest specialization.
Even if I accept your premise (which I don't), nobody is suggesting you apply the same mindset to two different things. I consider myself a full stack developer, but if I had to pick one it would be back end as that's where I started, where my strengths lie, and if I never wrote another line of JavaScript in my life I'd be pretty happy overall. But your characterization of full stack devs does not match up to my reality.