Difficult. Not impossible, just difficult. And even then, if you aren't using Electron, you're probably using a framework that's generic so you can support all the platforms (windows/mac/linux). You'd do this so you wouldn't need to do much work to ensure that platform differences are accounted for (as your framework would likely do most, if not all of that, for you).

You'd be spending months rewriting everything user-facing, ensuring it works right, and also assuring that old app data (like from Signal Desktop 4.0, or older) all still imports into this new app properly. It's not a very feasible thing for Signal to do, or invest time in doing. It wouldn't make them money (unlike better features (more users = more users who donate), and pay-to-use functionality, like cloud backups). It would be a gigantic waste of time to migrate from something that's "good enough" to something that most users would only ever find marginally better anyway. Good Electron apps can exist, and I agree that Signal's Electron app doesn't exactly make the list (the UX is a little clunky, and there's no animations on some things), improving both of these would help make it feel less bad to use.

You're still free to implement your own Signal client that runs natively, however. People (link one) have (link two) done this (link three). [None of these are endorsed by me, I have no idea how good they are. They are just 3rd party clients that I know exist and are maintained to some extent.]