There's more to a UI layer than individual controls. Like spacing between them fitting platform conventions, and window resizing and text size needs them to coordinate.
It actually is. Fortunately, the quality of apps dropped significantly, allowing these technologies to compete.
why should it? it makes no sense to me; you can do anything
It just might be that you don't go all the way, because you think a 80% sufficient result is enough for you
To the contrary. Developing native apps is so painful that the average S&P500 company will treat it like a root canal. Given how meaningless native functionality is to them, they won't hesitate at the opportunity to serve you a shitty Electron app. Many of them are only successful because they didn't waste time writing native apps for each platform.