Maybe because the Dutch don't do as many short hops as compared to the British? So the market is less price sensitive originating in AMS? Or if KLM still holds on to the Saturday overnight requirement, BA has no reason to lower fares. Just guessing...

If you book in advance, fares of €99 inclusive everything can be had on Eurostar (traveling part of the way on Thalys, now operating on high-speed track to Brussels).