A lot of desktop GUI toolkits were funded by the people who created the platform themselves so they had every incentive to make it both flexible and consistent.
When I look for a web component, the few component libraries that cover several different components are often extremely incomplete. Often they’re just side projects of a company or even an individual. No one is putting in the money to create something fleshed out.
On the other hand, when I look for just a specific component (like a drop down or autocomplete textbox), there are sometimes very complete but because they are a single component, it has its own usage and styling conventions that are going to be completely different from some other component in my app.
No one has the funding incentive of building a complete component toolkit for the web.
