Because you're reliant on the registrar and potentially a third party operator running the authoritative name servers? You're already reliant on those things, regardless of whether you want TLS or not.

I am a very big proponent of building things yourself (rather than using an off-the-shelf/existing solution that's similar) - whether that is tooling, or applications, or hosting your own services, and I'm particularly vocal about not making your business be shackled to specific services run by others (e.g. I'm of the opinion that offering exclusively "social login" is a huge red flag), and even I don't start complaining about "well this stack isn't really self hosted, you're still relying on the registrar for your domain!"