Comment by AdamJacobMuller

It's not even that it's not automatable, it's just that it follows a completely different control scheme and path than DNS.

for 99.99% of cases when a domain is pointed at me and I want to serve an SSL certificate for it, I can answer an HTTP-01 challenge. Needing to orchestrate a DNS challenge will always be a more complicated external thing.

HTTP challenge (and TLS-ALPN) are in-band, DNS is out-of-band.