Funny how nobody has actually addressed your question, choosing to praise rust instead..

The answer is that there are several python GUI applications (a dozen music players, Cura, Calibre, Anki, Deluge, etc). Hardly any Go because all bindings suck (which isn't that surprising, go devs are often hostile to cgo) and there's no (non toy) native toolkit either. I'd don't know why that is. Every few years I look for one but I give up and write the UI for my Go tool in html instead...