It used to be if you could find a solid engineer with some vague aesthetic sense they could efficiently carry something through from conception and design to completion.
These days the scope of frontend development has increased to the point where learning and staying on top of it has become too large (in my opinion) for most people to do while also becoming and staying highly competent in everything else.
A “full stack” developer these days is supposed to understand frontend development, backend development, systems administration, cloud operations, and what else? All of these are massive areas that are constantly shifting.
By way of analogy, we used to have the town barber handle haircuts and shaves but also surgery because the main skill to master was precision with a blade. Then as the scope of being a surgeon expanded to include things besides being able to cut precisely, that became its own specialization. Further on, even within surgery each area of the body started to require so much specific knowledge that we further broke that down into all the different surgical specialties (heart surgeon, podiatric surgeon, brain surgeon, etc).