IIRC, it's because MTO only designates provincial routes on roads they maintain (I think I remember something on the AARF about the border posts not meeting 400-series standards too). As such, the official end of ON 402 and the QEW are close enough to the final interchanges for 1PPI (and the same will be true for ON 401 as well once the Gordie Howe Bridge opens). Technically ON 405 should end at *PorRd for the same reason, but that change was never made.