They don't reveal these rather significant charges, and if they do, it's in such a small-sized print placed at such an awkward place that it's hardly visible amd hence doesn't qualify as "we fully informed all customers". These websites non-insignificantly increase your switching cost by first making you go through the entire booking process, including filling names, details, etc., and only after one has "invested" a significant amount of time/energy and is thus more "committed". They know that one is significantly less likely to abandon booking [upon seeing the onerous "convenience" charges in the end] once one has spent a great deal of time/energy on filling forms, etc. That's the psychology they exploit. Their revenues/profits aren't based on only delivering real value to their customers. Part of it also comes from pure deception and nefarious psychological tricks.