They have to implement the existing API or their own. The cost will be the same for both cases. For big platform vendors, almost no(extra) cost.
On the other hand, we, the ordinary developer must pay the cost of supporting N APIs on M platforms. Every APIs carelessly invented by vendors just because almost no extra cost burden us all.
Hence middleware engines, with productive high level APIs, while using every feature across all boards, without having to deal with messy extensions.
AMDN and Khronos are still fighting to get the love of CAD industry, adding back OpenGL features, because after having to create Iris Inventor like APIs in-house, they aren't keen in switching to yet another low level API.
On the other hand, we, the ordinary developer must pay the cost of supporting N APIs on M platforms. Every APIs carelessly invented by vendors just because almost no extra cost burden us all.