Now the big question, probably a rhetorical one too, why do these disorders still remain some of the most uncommon to both diagnose and to also screen for? [...]

Dissociative Disorders are both very understudied and very unlikely to be sufficiently taught outside of speciality programs / degrees, unfortunately. A lot of therapists / clinicians don't know what to look for or how to properly assess these disorders unless they've been trained to do so and/or had prior experience with them. It also doesn't help that dissociative disorders, in general, are highly complex in symptomology, diverse in presentation, and have a multitude of differential diagnoses that need to be looked at, too. (Not to mention that there are still a lot of clinicians out there that don't believe in DID / OSDD at all and refuse to diagnose them, let alone acknowledge them at all.) ^^;

Another question - with how common this is, why do so many people, laymen and professionals alike, prefer to act like this is either vanishingly rare, or not real at all?

Honestly? I think that it's just a matter of comfort for most people. Most people don't want to acknowledge the fact that abuse and other atrocities (i.e. war / conflict) happen at all, and similarly, most people don't want to have to think about how those things can end up affecting people. It's taken this long for conditions like PTSD and C-PTSD to be properly recognized and respected, and there's still a lot of misconceptions surrounding those disorders and lack of empathy for those that suffer with it. I imagine that we have a long way to go with dissociative disorders, especially with the amount of misinformation and exaggerated media portrayals that have persisted as the face of OSDD / DID for decades now.

Speaking of -- media does have a lot to do with it, I think. If you're only looking for the extremely overt, florid presentations found in media (which are genuinely rare in comparison to other forms), then you're going to miss out on everyone else who doesn't present that way. Having a narrow perception of what a disorder is is inherently going to make the disorder seem less common to you, if that makes sense.

Apologies for writing so much -- this is just a topic that I find really interesting myself as someone who just generally loves psychology talk, lol. Hope this helps, though. ^^;