because OSes have different UI toolkits with their own vendor lock-in, quirks, limitations, preferred languages, legacy baggage, and random bursts of "everything is now deprecated except the newest fad which will be deprecated next year" and "we're making everything flat and/or semitransparent!". Then most of the time the stock widgets are too basic to even support what Discord wants. This is an expensive PITA to develop and test for, and it's what lead app developers to run away to Electron.