That's because they... actually are domains too. Top Level Domains, as the acronym says.

It wouldn't make sense for TLDs like com, net, org, etc.

Actually it might --- that's a great place to put all the information about (and maybe instructions on how to register) a subdomain there.

But, to use the precise terminology, a domain name with a single 'label' has historically been treated as a "relative" domain, e.g. https://localhost , and no doubt many of you will be familiar with intranet sites that use similar "non-qualified" single-label names.