Flying and teaching are essentially different skills. There are many pilots who, while busy with line flying and familiar with normal procedures, may not have the luxury of understanding the intricacies of an aircraft's systems, nor do they wish to mentor unless they are pressed into a part-time job or retire. In most jurisdictions, there is no requirement after CPL (except for an FIR) for them to have hundreds if not thousands of hours before they can teach.
Looking at it from the other end, the MPL does not require any hours in the real aircraft (except a line check) before they are let loose as a First Officer in the right hand seat on commercial flights carrying hundreds of fare paying passengers. Even less, a type rated pilot without real flight hours should be allowed to teach in a simulator provided proper instructional techniques are learnt beforehand.
Agreed, time for a change.