Why DMARC failure (forensic) reports are not supported by major ISPs such as Google, Microsoft, Yahoo, etc.?

This is because failure reports provide detailed information about individual messages that have failed DMARC evaluation, which can include sensitive data such as message headers, visible From: & To: addresses, and the Subject line. Because of the sensitive nature of this information, many ISPs choose not to generate failure reports. Instead, they typically provide aggregate reports, which provide a summary of the messages that have been sent from a domain and how they have fared in DMARC evaluation.