Because sometimes it’s more convenient to point to someone else’s nameservers (I commented elsewhere in this thread about our AWS setup), and some people don’t want to give a single company control over their entire domain-to-web-server pipeline, though like I said I’ve never seen how that matters in practice.

Try using Cloudflare and having a wildcard subdomain and then letting others CNAME to it.

You cannot do this on Cloudflare without purchasing an Enterprise offering with SaaS features.

It's a very basic thing, allow CNAME to your subdomains... but nope.

So if you want to do this, you cannot use their DNS or Registrar.

Could be particular registrar && DNS not run by that registrar