> Why didn’t the DMARC spec say that a domain is “not present” if it lacks MX records?

MX implies a domain can receive email, you don't need it to send email. A setup where company.example sends email from companymailings.example but sets a reply-to for support@company.example is perfectly valid (even if it's stupid and confusing). Plus there's that weird legacy behaviour of mail servers delivering to port 25 to the IP in the A record if MX records are missing.