This is a terrible implementation of domain verification. dns-01 and http-01 are more or less standardized at this point. Use them, and don't roll your own. Reference: https://letsencrypt.org/docs/challenge-types/.
I don't get http-based verification in general. If you want to really prove someone owns a domain, make them change an authoritative DNS record. Everything else feels like it is begging for edge cases to crop up. Why should my social media or SSL certificate vendor care about my web servers?
I think people don't want to put DNS admin credentials in places where they might get leaked. Would be cool if a DNS server or provider offered credentials that could only do ACME challenges and not change any other records.
acme-dns[1] is probably what you might want if you are up for running your own bit of infra. Implements a simple rest api for changing the txt records for acme verifications and nothing more. It works nicely as a delegated nameserver.