I can't fathom why a regulator would try and stop a PPL from flying a heavier than 12,500 lb aircraft
I thought that must be sarcasm until I read your earlier posts, although the mass of the aircraft is largely immaterial to this debate, which is more about complexity. Most regulators require an ATPL to command a multi crew aircraft (i.e. an aircraft that is type certified to be operated by two or more pilots). If the reason is not obvious, it’s to do with safety (to those on the aircraft, but also those in proximity). There are plenty of biz jets and other large aircraft that are certified to be operated by one or two pilots, because they are designed to be safe to operate single pilot. The ATPL, and more recently MCC, are designed to ensure pilots of multi crew aircraft have a minimum level of competency in the technical and (more importantly) non-technical/CRM aspects of operating a complex, multi-pilot aeroplane.