When a CA issues a cert, the cost to the CA is essentially nothing.
When a registrar registers a domain, that costs the registrar money because the registrar has to pay a fee to the registry. So a registrar generally cannot give out domains for free.
Now, a registry could in decide to charge no fee. That's what .tk used to do. So you could get a .tk domain for free. Of course then .tk domains got the reputation of being cheap junk and spam.
Zero cost TLS certificates might be what Let's Encrypt is known for, but it wasn't the first entity to offer them. LE also disrupted the CA market because of IdenTrust's initial cross-signing, and ACME's development and rollout, under the banner of a well-funded reputable 501(c)(3). Nothing is stopping someone from starting an above-cost 501(c) non-profit registrar, similar to the ISRG or the PIR.
When a registrar registers a domain, that costs the registrar money because the registrar has to pay a fee to the registry. So a registrar generally cannot give out domains for free.
Now, a registry could in decide to charge no fee. That's what .tk used to do. So you could get a .tk domain for free. Of course then .tk domains got the reputation of being cheap junk and spam.