@ Feltwad
Because 75grs (2 3/4 dram) of ffg behind 1 1/8 oz of any size or composition shot is iffy on the larger game birds. Pheasant, grouse, dove, Hungarian partridge, crow, smaller ducks, quail are much easier to incapacitate and kill than geese, the larger ducks and turkey.
We have to use non toxic shot on waterfowl and many localities are mandating nontox for EVERYTHING. Steel shot is very low density and loses energy and momentum fast. Tungsten can be denser than lead but can be brutal to barrels. Worse than steel. Bismuth is a nice compromise.
If I recall you are from England. Not sure how much goose hunting goes on over there. Giant or greater Canada geese can weigh 15# +/- and are tough. They also can fly upwards of 70 mph. They cruise at 25 to 40 mph. The shotgun load that would fold a pheasant 100% of the time would do poorly on geese.
Mature Tom Turkey weigh 15 to 30# depending on sub species and diet. Very tough birds. Body shots are not taken due to feather deflecting pellets. The head and neck are the kill zone. This makes for a very small target at 20-40 yards.
Large ducks, all geese and turkeys make for a different hunting scenario than flushing pheasant or partridge with a good dog.