Especially because the last step is less "make it" and more like "npm add it".

The browser has "ready-made components". React (et al) also has "ready-made components". React's selection is a thousand times larger. React also allows you to make your own. The browser doesn't [1].

Who can blame developers for skipping the inconvenient half-solution and going straight to the all-encompassing ecosystem

[1]: It does, but web components are too unwieldy to compete with React