Sounds like some of this strife and frustration and criticisms being levied has more to do with organization / management vs why there aren't as many senior FE developers as compared to other specializations. I doubt the situation at your organization is the same everywhere.
IME most full stack people only know backend application development well, and the rest is knowing that the UI applications will make requests to the backend applications. Most self proclaimed full stack folks (again, IME) don’t know JavaScript well enough to build something good, and hardly anyone knows native application development. The only reason they call themselves full stack is they maybe used jquery a decade ago or have pressed F12 to see what happened in the network tab. Again, just my experience.
I take anyone who claims to be full stack at face value. More often than not, they’re not as full stack as they imagine they are. UI development is in my opinion much more involved than backend application development. Most paradigms on the backend have not changed in years, and most problems save database seem to be the same and already solved.
I've personally seen many (junior) FE engineers decide to switch to BE, "full-stack", data or devops (or even design) because of how messy FE can get.
I know it's a lot of things (organization, management, people issues, expectations, etc) and this is basically just what I observe in Singapore. Could be different in other cities/countries.