The public suffix list servers the purpose to only separate (sub)domains that are reasonably expected to be controlled by different owners.

Many systems - ex: rate limits, malware domain lists - would be very easily and cheaply gambled if domain owners could "disown" subdomains at-will, just with a change in DNS. There's a fairly long review process to get onto the public suffix list for exactly this reason.

There's also the historical aspect, that DNS is a much older technology than the need for the public suffix list. Mozilla at the time couldn't expect that all registries would adopt a new standard quickly or at all. Since there was a need for this information for browser security improvements, the list was born, and gradually become the de-facto standard source of such information.