Another reason is that these routes operate an airfare model where all tickets are for a specific seat on a specific service, with increasing prices as the seats sell out. It allows for more efficient train loading, with the downside that you can't turn up to the station and assume you can take the next train.
HSR is more towards airfare than metro-like train services though. The latter which is basically the Dutch rail situation: it's more of a national metro service than a method for voyaging. The fact that 95% of all travels are commuters underlines this.
Now I know some commute on the TGV, but the airfare model works really well for a lot of people, and I think it makes sense for longer journeys. Alas, the Netherlands doesn't have any (national) long journeys.
That said, Dutch rail has a discount system in off-hours now, where you can get sizeable good discounts if you book a few days in advance. You get a ticked valid not for a specific train, but a 4-6 hour timeslot.