I travel by train fairly regularly, almost every time I go there are lots of seats with reserved tickets in them. Very often the seats will remain unoccupied.
Some of the journeys indicated on the ticket would have been rather expensive to buy (for example, Penzance to London) and yes they could be sitting somewhere else but if you had gone to the trouble of reserving a seat by a window/table etc why would you not use it? Aibu to wonder why so many people seem to just not turn up?