Let's suppose that I'm a competitor of Google and Amazon, and I want to have my Public root CA for mydomain.com to offer my clients subdomains like s3.customer1.mydomain.com, s3.customer2.mydomain.com,...
If you want to be a public root CA, so that every browser in the world needs to trust your keys, you can do all the lifting that the browsers are asking from public CAs.
Why do you want this when there are wildcard certificates? That's how the hyperscalers do it as well. Amazon doesn't have a separate certificate for each s3 bucket, it's all under a wildcard certificate.
Amazon did this the absolute worst way - all customers share the same flat namespace for S3 buckets which limits the names available and also makes the bucket names discoverable. Did it a bit more sanely and securely at Cloudflare where it was namespaced to the customer account, but that required registering a wildcard certificate per customer if I recall correctly.
The only consideration I can think is public wildcard certificates don't allow wildcard nesting so e.g. a cert for *.example.com doesn't offer a way for the operator of example.com to host a.b.example.com. I'm not sure how big of a problem that's really supposed to be though.