They did, they were called "Google Play Editions".[0]
No one bought them, because they were released at a time when people still paid subsidized phone prices and upgraded their phones every two years for $200. So when they were asked to pay $600 upfront, the vast majority had sticker shock.
> I don't understand why manufacturers simply don't offer a stock Android variant.
When the hardware is commoditized, manufacturers need to distinguish themselves by offering a unique software experience.
That's what they are all trying to do. Unfortunately, most of them aren't any good at it, and the UIs they come up with are usually worse than stock Android. But if they offered stock Android, they would have to compete on hardware alone, and they are just slapping commodity parts together like everyone else except Apple.
My android experience has been tarnished more by Google bugs than Samsung ones. Android 5 was a very buggy release, so much infrastructural change with little user benefit. IMHO Samsung should have skipped it entirely. I guess they bowed down to user and media pressure to always ship the latest android (although the media criticised them anyway for poor battery life, caused by Android 5).
Off the top of my head: Knox secure folder, better split screen options (app pairs), app suspension, power saving modes, file manager, better camera app, many more configuration options.
I don't understand why manufacturers simply don't offer a stock Android variant.