The problem there (and this is from experience building and supporting such a tool for non-technical users) is that people learn. Anyone who spends enough time in one of these tools will quickly pick up the fundamentals enough to become frustrated with the limitations. Or their engineering team will see the inneficient code being created and take it upon themselves to just implement these things quickly and simply. So you can start leaking the abstraction and allowing custom CSS/HTML, but then you very quickly arrive at a place that is useless for both teams.
There is a place for both. You don’t need to spend much time on here, reddit, or many other forums to find people complaining about the lack of decent middle ground tooling to get started with front end dev.