Because there is no effective consumer protection in the rail industy. The TOCs know this and are taking full advantage.
They're also (successfully so far) holding up the implementation of the promised consumer protection body which is now several months overdue (according to the initial "comittment" from mid-2017, the ombusman was supposed to be "expected to start early next year". We're well past that point now. Presumably they're dragging their feet in an effort to get the idea dropped.)
If I responded to every inaccurate assertion posted on Railforums I would never be off the site but this one is so far from the truth that it does need to be corrected.
Firstly, this is in no way a defence about the reason for the OP - train company staff shouldn't be giving incorrect advice about ticket validity.
However, in terms of consumer protection, it is Government that makes laws and it is Government that has to change those laws. The rail industry is subject to the Consumer Rights Act, so no different to any other service provider, but in terms of adjudicating for customer disputes, the rail industry is still subject to the legislation that mandates the existence of the Rail Passenger Councils (Transport Focus and London TravelWatch to you and me). The use of these to adjudicate disputes is out of date but to formally replace these with an Ombudsman requires legislation. Because there is no prospect of such legislation being passed in the next few years, the industry is voluntarily setting up an paying for an Ombudsman scheme. This was announced by the previous rail minister but that was on the back of the industry agreeing to set it up and fund it.
Believe it or not, establishing that scheme within the legal framework of the current obligations is extremely complex and is taking time. If it is going to work properly it needs a lot of care - especially as the industry has to fund it (because the DfT has no money to pay for it) but it needs to be structured so as to operate impartially. We have seen elsewhere what happens when a complex change on the railway is pushed through too quickly without proper resourcing.
By the time it is set up it will have taken around 2 years from first suggestion, which is around the time it was always going to need from a standing start, regardless of some of the pronouncements at the time made by people who do not actually have to plan and make these things work. Please do not confuse 'dragging their heels' with 'lets do it properly'. And just to repeat the key issue - the Ombudsman scheme is a voluntary one being funded by the industry.