Why are they so hostile to Vulkan and what problems does Vulkan, which is vendor independent and open, have that adding a new API to support to the pile would not?
Lack of known good reasons, and their major infamy in perpetuating lock-in in general. They have only themselves to blame for being seen in negative light in this topic by default.
That doesn't mean their isn't good reason. I guess they believe they can build a better product by having control over the graphics pipeline. Obviously I cannot say for sure what they think in that regard, but neither can you.