Why? The high sensitivity domains that are most likely to want to be verified via the (mildly confusing) domain-to-user mapping on Bluesky already exist. To take your example, Apple is not likely to try and register iphone.apple.com on Namecheap.
On the other hand, "normal" users who just want to chat with their friends are not going to care, and might actually be a little confused by this announcement. "Wait, I need to buy a domain to use Bluesky?"
Now, I could be wrong, and Bluesky might be trying to get a bunch of high-flying corporate accounts on board first. In that case, they are doing the right thing, and I look forward to when large media orgs start posting on Bluesky.