It’s not. There’s LetsEncrypt, ZeroSSL, BuyPass, SSL.com, and Google Trust Services[0]. The ACME protocol is standardized and you can point your client at any of these at any time, and other providers can begin providing certificates at any time. Some tooling[1] even uses other providers by default.
[0] https://acmeclients.com/certificate-authorities/ [1] https://github.com/acmesh-official/acme.sh/wiki/Change-defau...