GUI designers of the RAD era were much easier than Photoshop or Flash, and certainly easier than writing it out by hand. That's precisely why they became so popular.
The most popular frameworks with the best tooling weren't cross-platform. VB, Delphi, .NET/WinForms (and later WPF) were all Windows-only. Delphi briefly played with Kylix, but it was very messy.
Then you had Qt, which is a very Delphi-like take on GUI in C++, and was cross-platform - but it didn't have the integrated tooling on par with the other stuff, and of course the language being C++ raised the learning bar significantly.
Qt is much better these days, thanks to Qt Creator - although C++ is still a stumbling block there. But in the meantime, web apps took over - and that was before stuff like React. I don't think that had anything to do with ease of development, but rather with ease of deployment (or rather lack of it) - it's much easier to get people to use your app right there in the browser than it is to have them download and install it. So web won not because of its technical superiority, but despite it - and then dragged the desktop down (Electron etc) as a result.
I remember in the 90s there was a push for putting things on the web to avoid licensing fees and installation, despite how limited html and JS were. And there was the temporary popularity around Java applets to provide a more rich web experience prior to Flash and then finally HTML 5.
