If a TOC only checks tickets 50% of the time, then you have to pay the daily fare 50% of the time which might not be much of a saving on a season ticket, also that will only really work if you are both boarding and alighting at an unstaffed station which will not be the case for many commuters. Sure, you could "short fare" at a large terminal station by claiming to have boarded at a station closer to your destination but occasionally there are stings carried out at stations where the number of claimed ticket sales exceed the number of observed passengers, and genuine passengers will be sold tickets/given documentation by roving RPIs. Do you really want to be scanning the platforms at, say, Burley Park every single day to see whether there are any signs of rail staff if you are thinking of short faring? I wouldn't think it worth the worry. Also bear in mind that whatever you try, you might get away with a "penalty fake" charge the first time you're caught and think it balanced out, but the next time they may go for a prosecution instead which will cost you much more.
I'm sure the TOCs have thought it out and, yes, some journeys might evade any kind of ticket check on a regular basis and if you can board at a station with no opportunity to pay and alight likewise then you're in the clear, but these flows are likely to be very small and they cover the vast majority of regular travellers by the ticketing arrangements they do have.