Except GTK looks like shit on Windows and Mac, so nobody wants to use it for those platforms. Most developers that create software with these bloat issues would not accept GTK.
Is it because they’ve not been updated to use the latest UI guidelines from the host system? Because they just look old and out of context, not particularly ugly.
The truth is, Electron is easy to use, quick to hire and cheap to build on, that makes it the best choice for many companies, even when the user might (highly-likely) hate it.