Those are simply the rules. You can do ACME with an HTTP challenge or a DNS challenge. The HTTP challenge is adequate for proving that you control x.example.com, but serving a website on x.example.com doesn't prove that you own y.example.com. But, being able to create example.com DNS records does, so that is what's required to get a wildcard certificate.

I imagine you are confused because the proposal above sounds like "just get *.example.com, then copy that cert to everything that will ever serve traffic for example.com", which doesn't sound like a great idea to me.