Because they haven't tried it. Full stop.
And when looking at it side by side with a magnum cartridge it just doesn't feel like it has the wallop factor. All the "margin" talk fails to account for the fact that accuracy nosedives when shooting from field positions, and bigger cartridges recoil more than smaller ones, exacerbating non-perfect positions. A lot of people don't realize how much the rifle moves before the bullet exits the bore. Magnums move more than small cartridges, period.
