My understanding is (and I have experienced this myself), on peak use trains, they are so over-capacity and under-staffed that conductors don't have time to check all tickets between stops before they have to run to the doors, or there's so much crowding they can't even make it down the hallway, meaning that some people could theoretically not buy a ticket and get a free ride in cramped, unpleasant conditions.
Clearly, the best solution imaginable to rectify this scenario is to make sure everyone boarding at South/North pre-pays just in case the conductor can't reach them (but of course this only affects boarding from that one station so conductors will still be assigned to try to check every ticket since you have no way of knowing who boarded from where).