"People are wondering why developers aren't taking advantage of AMD specific features."
But that is only the PC part of the story is it not? Developers of major titles still tend to develop for consoles first and farm the PC port out to a secondary studio (see Arkham Knight, Mortal Kombat X, Destiny 2, etc.) Consoles currently do run AMD hardware, so one would expect that the studios would optimize for the Xbox/Playstation hardware.
But here is when things run afoul, likely exactly how you suggested in your original post. The secondary studios tasked with giving PC gamers a good experience probably could do a near direct port of the Xbox version to Windows 10 and have a perfectly functional product that could be scaled up in resolution. But NVidia does pay hefty sums to lobby developers into adding NVidia specific "Gameworks" features. Those additions can break functionality on AMD hardware, or at least run less efficiently, ensuring the PC port will run better on NVidia and push PC gamers into purchasing NVidia hardware to get the best frames.
Some have argued that AMDs rise as the choice for miners, would also help it in the PC gaming space. AMDs superiority in mining and "good enough" PC gaming performance would make it the choice of miners who also game. The numbers you present dispel that illusion. In the time frame that Vega 64 has been on the market and also sold out, the RX 580/480s have also been impossible to find. Someone is buying all those AMD GPUs. And yet, when you look at Steam, a significant sampling of the overall PC gamer population, the penetration of AMD GPUs is actually decreasing at an accelerated rate. So all those cards aren't being logged into Steam, and that trend as you say shows no signs of reversing.