This only became a problem because of the introduction of open stations, where you can just walk onto the platform, often without directly passing any ticket-issuing facilities, usually because the ticket office or platform access was not relocated when the station was made open. The onus of buying a ticket was then passed to the passenger, without any sustained public education to back it up.
I suspect that even in this day and age, most people's source of information when making their first rail journey will not be the internet, but discussing their plans with friends and family. And if the friend says, "just get on the train and pay the guard, that's what I did when I last travelled, and I had no problem", that's what they'll do.
However, the biggest problem in my view is the normalising of paying on the train, by guards who just sell a ticket without telling the passenger that they have done anything wrong. I fully sympathise with people who have come to think that paying on the train is acceptable practice through this, and suddenly get caught out by an RPI. Yes, I know that strictly speaking ignorance of the law is no excuse, but this does seem to be laying a trap for people to fall into.
In addition, most people's first experience of public transport is usually the bus. If you routinely catch a bus at a bus station with a ticket office, it is usually perfectly acceptable to pay on the bus, so why wouldn't you assume that the same applies at a railway station with a ticket office? Likewise there a places where ticket machines are sometimes provided at bus stops to speed boarding, but it is still perfectly acceptable to pay the on the bus.
The first time my sister travelled from South London to central London (in pre Oyster card days) she only realised that she had to get a ticket first before boarding because there was a man in a booth who wanted to see her ticket before she could get onto the platform. So when she got off the train at Victoria she then bought another ticket for the underground. And then when she changed tubes at Oxford Circus, she went back up and bought another ticket for the central line! She hadn't realised that you could buy one ticket for the whole journey, she was just so used to having to buy a fresh ticket every time she changed buses, she just assumed the same applied on the railway.
So, my answer to the original question "is it obvious that you need a ticket before boarding?", is no it isn't, not unless there is an obvious barrier line before you get to the platform.