Maybe because Web development is dominated by FOSS tooling and as such very few companies are willing to invest the money to pull it off, as very few would be willing to pay for such tools.

Then there is the whole Web foundation that makes it pretty hard to actually use layout designers and components in a way that can work across frameworks and browsers.

Only now with CSS Grid and WebComponents is the Web finally catching up with what was already possible in desktop framework engines of the late 90's.

There are a few startups working on these ideas, but again, the main question is how they will manage to stay alive.