afaik not a problem with apps written in something like react native which use platform specific widgets and common business logic.
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.
Accessibility, text field behaviors, control animations looking right, window resizing behaviors, any system settings related to UI applying properly, printing, export to PDF, rich text copy and paste, normal power and memory use, the engineers working on the next OS release being able to fix your app if they break it, your app updating to the next OS version's UI changes automatically…