It's pretty crazy that they currently have the power/leverage to demand that of OEMs.
Indeed, early on in Android's life, Google was much more hands-off with the OEMs, letting a thousand flowers bloom... we got fragmentation and subpar devices, developers and consumers complained, and Google set forth on tightening the reins and exerting more control such that when people buy an "Android" phone, it means a particular experience. (And of course there's nothing stopping OEMs from shipping AOSP-based devices and just not calling them Android - as in fact a number have done).
The issue I have is that if you're a non-techy user and you get accustomed the "The Samsung Way", then try to switch to a different brand device, it can be very disorienting. They practically feel like different OSes in some cases.
In addition, the current Nexus devices (5/6) are pieces of crap compared to the halo phones from the big OEMs.
But I do agree, that OEM skins do improve some lacking features of the stock Android, they also bring in tons of dark patterns and just terrible UX design with them - TouchWiz (Lollipop) for example removes all timed audio profile functionality, brings confusing duplicated apps for pretty much all standard applications (Play Store, Google Fit, Calendar, Mail, Keep, etc.) with their own separate accounts, hides camera functionality in public API and several other problems. Not to mention a completely different design of the OS from the whole Material ecosystem.
And thats a tip of the iceberg considering the times of Android 2.3/4.0 when Google CTS tests weren't so strict and developing anything worthwhile was practically impossible since OEMs kept overwriting even default integrated themes - stuff like setting black text to be white, removing fonts and other fun stuff we had to deal with.
Samsung phones have been degrading in quality as well - screens have become more fragile, Samsung has focused on copying Apple in many ways, and probably most damning, the market has responded negatively to these more recent changes.
Give me a more spartan device like the Nexus 5 over that crap any day. Google had it right, but unfortunately has been blocked from bringing that experience over wholesale.