Because the hash comes from the same source as the image itself.

So if someone compromised the publishing infrastructure (or does a MITM attack), they can trivially maliciously modify the hash as easily as they can maliciously modify the image. It provides zero security to Publishing Infrastructure compromise.

A good historical example of this happening was when monero’s release infrastructure was comprimised once. And here’s a great list of historically relevant cases where this happened:

• github.com/cncf/tag-security/tree/…/compromises

In the case of Monero, the users were able to verify that the cryptographic hash on the release was invalid, and it was fixed very fast. If the user can only check checksums, they have literally no way to detect if the publishing infrastructure or a MITM attack is taking place.