I am fascinated by this. I guess when there is no universally recognized ID it feels weirder?
I mean, sure, by all means withhold info from social media platforms, but if it's one where you're going to have your real name and your whole-ass work history on public display, surely verifying your ID is trivial? You could absolutely google the info in a LinkedIn page and find a bunch of additional info anyway.
I get it intellectually, it's a taboo now, just like it's a taboo to have people find out your address or phone number when it used to be publicly listed until a few years ago. It's just weird that it's still a taboo for the services where verifying your ID is presumably a feature, not a bug.