My pet theory is that they were a form of marketing for the folks who grew up with the wild-west-internet. The internet now is more corporate and "serious". You have an entire generation of workers who only ever knew the internet in its walled off / corporate form. And then you have folks want to "just get work done". So you'll continue to see fewer of these harmless chicanery.
Google's still got it. This year they announced they were purging data they vacuumed up from Chrome users in incognito mode. Oh Google you little scamp!
Otherwise, we refer you to the tongue-in-cheek Financial Times 'why this page was not found' rationale: https://www.ft.com/errors/page/404