In a lot of cases, passengers believe it's the train companies responsiblity to sell tickets, not the passengers responsibility to buy.
Sometimes you literally cannot get through the train systematically to pick everyone up. There's also the obvious toilet hiding, doubling up, using a Z1 or short distance season to pop barriers.
There's also a lot of groups travelling around the network intentionally fare evading, and in some cases bragging to railstaff, as if they expect them to confront them in some form. No one gets paid enough to make assult acceptable, and if you've got that mantra and personsality, then by all means crack on.....
I came across that recently. Someone had been hiding in the loo and as we neared the final stop, Guildford, they went into the guards booth, this being a 450 and said some rude words down the mic, along with a thanks for the free train ride. He claimed to be at the bavk but was in fact at the front. The guard did check tickets but there wasn't time to go back down. Earlier in the journey the guard would have had to fight through the passengers to check tickets, this being a Waterloo to Guildford via Woking stopper, which is always very busy with commuters until about Walton on Thames.
The person then walked down the ramp and I don't know what we did next. If he went to the front of the station he would have hit barriers. I did once see someone jump over the side though.
If he went out the back, via the footbridge, he'd meet someone too, with no side barrier to jump over.
However if he exited at the commuter entrance, only open during the week day peaks, he'd have potentially got away without paying. I don't know how well he knew the station layout.
Personally I think he should have just gone to a fares workshop and figured out the routing guide. OK he might not get free travel, which is illegal but at least he'd save money, which if you play by the rules, is legal!