LetsEncrypt is not the only ACME provider, and there are hundreds of regular CAs. Nothing about TLS certificates is centralized, there's just market concentration as a result of good UI/UX by LetsEncrypt, but there are open-source ACME implementations available, the protocol itself is in the process of being standardized and nothing keeps other CAs from running the same service, and in fact many are planning to do just that.
You're right that there are additional ACME providiers, but the reliance on just a handful of default root cert stores is what makes HTTPS centralized, even if TLS isn't.
