While what you say isn't wrong, the key difference is that in those circumstances the infrastructure is already there to run unprofitable services, so the only cost is the difference between the farebox revenue and the operating costs.But we run lots of train services that aren't financially profitable because they provide benefits in other ways. Why wouldn't this be the case with a reopened line !
However there does have to be a business case to open a new line which will cost a large amount in capital expenditure. I accept that the business case might not be entirely financial - an example of this is Okehampton/Meldon to Bere Alston which would provide a diversionary route to the coastal line at Dawlish, but where the finances don't stack up, a government would not support it. To spend hundreds of millions of pounds on opening a line which would not even cover it's operating costs would be insanity.