They have a graphics library inside of a larger engine. And for 99% of devs they don't care about directly modifying such code, so they are fine with the stipulation of "use our tool to get graphics" approach.

Didn't Unity and Epic have people working directly on the Vulkan specification? I remember reading a comment (maybe here!), way back during Vulkan's original release, where someone was screaming about how Vulkan was a conspiracy to make us dependent on giant, multi million dollar game engines.

I don't know about any conspiring, but I've thought about that comment often, because the result appears the same: The barrier to in-house game engine development has been risen further.