Ex-military jets are flown on a permit to fly and have an individual training course approved by type that each candidate undergoes. This then results in a type rating. The content of each course is judged on the operation complexity of the type. The JP as an example is a very simple VFR only permit aircraft.
A Lear as a complex IFR aircraft, pressurised etc. So the authorities have deemed that it requires HPA or ATPL/ATP level knowledge. The HPA elements really just look at human factors and performance and high altitude weather etc.