The problem is that accessibility has been treated as an afterthought, so designers, UX and people crafting HTML usually are not in habit of embedding accessibility into their works. It often comes when someone disabled complaints or if there is a regulation they didn't know about and there is "oh sh..." moment where accessibility is tacked onto already done project.
So it's difficult in the sense that people don't think about it, there are not so many tutorials available and so on.
YES. I think it's this unique form of after thought, where people imagine accessibility as a "score" that you can "increase". The web developer lowest-common-denominator cares about it, but only at this weird surface level. More aria tags = more accessibility points. It becomes this weird game where we end up with insanely noisy markup, which creates a horrible described interface for anyone actually using assistive technology.
It's an interface like any other. We'd be weirded out if designers graded their screen specs as "visual" enough. More pixels are required in this area, think of our screen users! They need more pixels!
So it's difficult in the sense that people don't think about it, there are not so many tutorials available and so on.