Because sometimes it’s more convenient to point to someone else’s nameservers (I commented elsewhere in this thread about out 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.

This implies you're using their reverse proxy feature, not just plain DNS records.

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