Thats not to say a layperson can't build a house, but having experience and training about a specific domain can make later modifications and updates significantly easier.
You don’t get back to architect when you’re doing plumbing. You don’t get back to plumbing when doing electricity.
And not only those are often highly isolated tasks but those domain have specialist in it as well. You might go to electrician who can do high voltage installations and you don’t care because he can to general wiring too (thus being full stack).
But there’s different about sacrifice that’s bigger issue. In software engineering handoff costs can be absurdly high. Teams can be allocated and if backend team misses on deliver the next window of opportunity can be in couple weeks. In micro, highly agile teams it’s yeah whatever, but I’ve yet to see a dedicated team of frontend/backend that’s on standby for the other party.
Its a short term productivity boost sacrificing for long term maintainability and future productivity.
I would take 1 backend engineer + 1 frontend engineer over 3-4 "fullstack engineers" any day of the week.