Because you already paid for it once!
You can buy powder, pre-primed brass, and projectiles from 308 and 50cal, which has been demilitarized (disassembled and the components harvested) but you cannot buy intact and functional boolits...
Think about that. Military ammo is purchased at milspec pricing from manufacturers at taxpayer expense, stored and managed at taxpayer expense, declared obsolete and sold for pennies on the dollar to salvage operators who commit to demil it, then offer up the components for sale to american taxpayers.