You'll have to ask Thalys. They set the fares, including the pass reservations.

There are several cases more-just reported here-for other superfast trains-of course all organised by what is effectively the French state Rail, that the surcharge is higher as a cheap advance ticket booked long before. Same more or less may also apply to EuroStar to LON. IF one also adds the average cost of a travelday (mostly 30-40€) the difference is even higher.

Often the surcharge alone is higher as even a prebooked cheap airticket for a longer distance -over that same railsector.

Answer should not be over-difficult to think of yourself: these are not heavily subsidised local trains but have to make money-and the cost of building these new lines etc is astronomically high. Things that all those green promoters always neglect to mention