Why?
I want my macOS apps to use Mac-native UI elements, and Windows apps to use Windows-native UI elements. (There is no one native Linux UI, so we can leave that one aside...)
Most of the users of the FOSS software I'm developing also switch platforms depending on the project they're in, but they want the software to be the same everywhere for instance.
As annoying as it is to switch form cmd+C to ctrl+C when I go from Mac to Windows, it's even worse when a Mac app uses ctrl+C.
Branding teams are in my experience very picky about lots of stuff. For example, my employer's brand design team recently released a rule that when our logo is included alongside others logos in a list of companies involved in a partnership, it may not be included in a vertical list, only a horizontal list.
Using the standard background colour for the platform on your app when it's not one of the brand design team approved colours? Also out.
etc. etc.
It's a common concern to avoid confusing users who switch OS often (eg when demoing on someone else's machine.)
The fact that it contradicts the other requirement ("but it does not look like the other apps") is lost on no one.
See VSCode, Discord, Spotify...
It's easier to get a single code-base that 'just works' across platforms if you have an abstraction that is truly cross-platform. One of the major value propositions of developing with Electron is this ability to develop cross-platform 'with confidence' - I can build an app on Windows and be 99% sure it's going to look function the same on OSX / Linux.
> I want my macOS apps to use Mac-native UI elements, and Windows apps to use Windows-native UI elements
That's a fine goal but it means you end up needing more platform-specific code. It's a trade-off, and there isn't a one-size-fits-all solution.
To take Discord as an example, I don't know that people actually can't live without the UI slavishly copied for every platform; I think people rather want light and dark themes on all platforms, message formatting to look basically the same, and for everything to be found in roughly the same place.
This idea that users expect apps to look the same is, I feel sure, something someone said and everybody has gone along with. In my experience, people want things to behave consistently — so if Discord had a native Windows client and a native macOS client, people would be more interested in whether or not they could still find the little microphone and headphone buttons at the lower-left of the screen, sod if the icons and colours are identical.
But I suspect lots of software companies would think that describes them, when in reality there's probably fewer than 10 products that meet that bar.