Why do people avoid paying? Sure, there are dishonest people who will evade at every opportunity. But there are also many others who wouldn't normally evade a fare, but for whatever reason choose to do so.
Perhaps somehow they feel that the railway owes them in some way. Perhaps they were seriously inconvenienced by a cancelled, delayed or overcrowded train and the 'delay repay' did not come close to compensating their resultant consequential losses that the railway wouldn't reimburse. Maybe they had to stand for several hours because the previous train didn't run as 'a member of the train crew was unavailable'. Delay repays where the claim is refused because the train actually arrived 30 minutes late, but the system recorded it passing the last signal into the platform at 29 & 1/2...
Or maybe they were screwed by the railway for an innocent mistake. You only have to read 'disputes and prosecutions' every day to see what I mean. Loads of people there going 'Yep, I screwed up by short faring, used someone else's pass, whatever'. But far more people are coming for advice on having been assessed an inordinately high penalty for completely innocent circumstances and the railway has taken the worst view.
Long queues, delays, unable to buy on the train, tried to buy at destination, suspected of fare evasion and fined. Genuinely mislaid purchased ticket, made to pay again, or made to pay the full single fare when they previously had an Advance. People will think, well, I paid £80 more than I should have, so the railway owes me that £80 back, if I can avoid a few fares, we're all square.