The reasons why Apple still maintains their own browser engine have to do with corporate prestige and expensive insurance policy. They can afford it because Apple is literally the world’s richest corporation.
Nobody else does anymore. Even Microsoft gave up on their own browser engine despite having 97% market share two decades ago. And Mozilla is bankrolled by Google as their own kind of insurance policy.
I'm quite certain there's no human on earth that could reliably predict the outcome of most non-trivial real-life CSS layouts given just the code. And even though the situation has improved a lot, it's still common to have to use clearfixque hacks that definitely weren't envisioned by the commitees.
Yes, CSS is a lot more powerful than those kinds of systems. Perhaps comparable to constraint satisfaction layouts that tend to be quite surprising.
But even so, there are so many settings and their combined behavior in CSS that it's, for me at least, totally infeasible to try to code much CSS "up-front" without constantly checking the result of the changes. And I don't mind it that much, but I'm quite certain that a more "predictable" equally powerful layouting system is entirely possible.
It could be an interesting challenge to show people some layouts and ask them to produce them using CSS without being able to iterate the results.