Aren't Vulkan, Direct X 12 and Metal much closer than Direct X pre 12 and Open GL ever were? The three are derived (at least in philosophy) from AMD's Mantle, not like Direct X pre 12 and Open GL that are (or at least were) very different beast.
I only ever programmed with OpenGL 1.3 and 1.4, so my knowledge is not very up-to-date to say the least...
Porting complexity is still non-trivial, and then you need people who understand multiple bare metal APIs.
The architectures are all based off Mantle, but OpenGL 3.3 and DX10 were very similar in their hay day, and porting was still fairly limited (you can still partially blame the horrible adoption of the 3.x series that took so long to take off).
I only ever programmed with OpenGL 1.3 and 1.4, so my knowledge is not very up-to-date to say the least...