So the question should be, why don't they put purchased duty free in the checked baggage to be collected at the destination?
Because traditionally duty free is purchased after check-in, airside.
A better solution would be to sell duty free only on arrival. It would solve a lot of problems :
- People holding up departures because they are buying duty free
- Fights about overhead locker spaces when people try to cram all the stuff in
- Idiots trying to drink duty free in flight
- The weight carried by the aircraft (2 x 1 litre bottles per adult on a 180 seat aircraft, say 150 x 2 = 300 bottles at 1.5 kg each = 450 kilos.)