For those in the industry who cannot see it, reasons why pax bring so much on board are simple :
1) 5% is lost by the carrier.
2) Sometimes 25% if a connection is involved.
3) The remaining pax have to stand round and wait and wait at their destination for the bags to be delivered.
4) Procedures that require, for example, on US to Africa flights, that baggage departing the US must be unlocked, or else the TSA break it open, while on arrival in Africa unlocked baggage is routinely rifled and contents stolen. And Heathrow is not much better in this respect.
Point 2 is probably an underestimate for KLM connections through Amsterdam, the start of this thread.