Because changing the train is completely different. You are moving from a train priced in a certain way to reflect the expected demand onto another which may have a completely different demand profile. Moving trains *does* have an effect on the TOC in a way that getting on a station later doesn't.
It is worth pointing out at this stage that, officially, changing seat reservations on the same train has an admin fee attached, though the actual journey is unchanged.
As a minor side question to anyone who knows (for which I genuinely do not know the answer), when you change the train being used, does the quota for the original train go back on sale?
....You can probably count on one hand the number of people who, faced with an Advance Ticket at £20, thought 'You know what, my car might break down or I might find myself called away to a town near the station one stop further on. I'll buy the £100 ticket instead'. It would be cheaper to buy TWO Advance Singles, one from each station you'd maybe perhaps possibly use, than buy a fully flexible ticket for that reason!....
Possibly, and that could be a better option in some cases, but as I noted, that is life and we have to accept when things don't go to plan.
When I book hotel rooms I reserve the cheaper non-refundable ones and I know that if I can't use it, that is tough, I know also that I am unlikely to be able to change the booking, that is also tough, but I accept this as a trade off for price. If, for some unforeseeable reason, I can't get there, I chalk it up to experience. I could moan and b*tch 'til my heart stops, but I know I made a choice and I have to live with it.
....Fully flex tickets offer far more flexibility than somebody in that scenario would ever need. They are happy to be tied to a specific train....
That doesn't negate the options for the passenger, the railway shouldn't be held accountable for a passenger's decision.
....Do you not think the fact that its often cheaper to travel further is the bigger problem than the myriad of rules we need to make sure people can't travel short distances for the same money as people travelling long distances?....
So you would rather see the end of Advance fares than accept a few simple rules? I can think of many people who would disagree with that point of view.
....For this particular example, jack up the price of the Rugby ticket. I had a £7.50 myself from Birmingham to London. Very grateful of it but it's a fundamentally stupid system that allows me to travel from Birmingham to London for CONSIDERABLY less money than somebody boarding my train at Milton Keynes also travelling to London!....
However, the Milton Keynes passenger has much more flexibility for that extra cost, they could, for example, travel from Watford Junction or Bletchley at no additional cost if they needed to.
....What? It's dead simple. Advance ticket is only valid on the train specified. This then prohibits BoJ because your ticket stops being valid once the train specified leaves your break point without you on it!
And what of journeys where a change of train is involved? (a ticket doesn't have to be '&Connections' to have a booked connecting train) For example, Stalybridge-Preston, 'rte AP TPE Only', a change of train is required at Manchester Piccadilly, in your world I could instead travel from Piccadilly or Bolton, equally I could end my journey at either, but I could not, for any reason, spend my 50 minute connecting time (example) outside of Piccadilly station? That doesn't sound fair to me and in any case it's not like I wouldn't have paid to travel the whole way anyway, I think it's terribly inconvenient rule and clearly not reasonable or fair and should be scrapped immediately.
Fortunately though we don't have that situation and the passenger can choose a more flexible ticket to suit their needs without complex restrictions on break of Journey, though it will cost more.