Email currently uses an HTTP request to https://mta-sts.<domain>/.well-known/mta-sts.txt, per RFC 8461. Depending on HTTPS/TLS instead of DNSSEC is one major reason you see this approach gaining popularity.

Id think its for enduser flexibility and convenience. Its more involved to alter DNS records vs just adding a text line to a webpage and hitting that default endpoint as standard. Same way LetsEncrypt allows the easiest setup with the .wellknown endpoint , where a user may not have DNS access beyond the initial A record setup.

It's not a good idea, I'd call it terrible in fact. We have MX Records, might as well use those. If not, use SRV records. Requiring mail server to involved in root of the domain is terrible idea.