The thing you're missing is that in this case UI is not something you can usefully make different without harming yourself in the process, due to it being a huge contributor to function beyond form, and "ripping it off" doesn't hurt anyone else as long as you don't actually impersonate them by copying their logos.
Gitlab's UI has evolved considerably in the last couple of years. If it looked just like Github when you last saw it, I would suggest checking it out again because it's quite different compared to the gitlab of 2013ish.
also I agree with you that stash and bitbucket are Really Not That Great.
I mean it's not like someone's breaking into GutHub's server room and stealing equipment.
You want to say it's a breech of copyright, patent or trademark? Maybe.
But then FreeBSD "stole" Unix, Windows stole Mac, Mac stole Xerox, Microsoft stole WordPerfect, and so on.