Think most of them use WordPress, which may or may not fit into people's ideas of page builders. tbh most of the OP's criticisms about composability and maintainability and feature completeness don't really matter for five page sites anyway, but an agency using a CMS template likely has the advantage of having some idea how sections and inheritance etc work, and more design skill in creating and using building blocks than the DIY-er.

And if you actually need something hand rolled using authentic vanilla HTML and artisanal node frameworks you can still get it for less than $200k :)