But they don't actually differ, see the "no graphics API" blog post we're all commenting on :) The primary difference between mobile & desktop is performance, not feature set (ignoring for a minute the problem of outdated drivers).
And beyond that if you look at historical trends, mobile is and always has been just "desktop from 5-7 years ago". An API split that makes sense now will stop making sense rather quickly.
Different features/architecture is precisely the issue with mobile, be it due to hardware constraints or due to lack in deiver support. Render passes were only bolted into Vulkan because of mobile tiler GPUs, they never made any sense for desktop GPUs and only made Vulkan worse for desktop graphics development.
And this is the reason why mobile and desktop should be separate graphics APIs. Mobile is holding desktop back not just feature wise, it also fucks up the API.