I don't know why they aren't required to list the powder and charge weight on factory ammo.
The main reason they don't list the powder and charge weight on factory ammo is practicality. Putting that info on the box is just a waste of the cost of putting that info on the box.
The reason it is a waste is simply because that information is
MEANINGLESS to the shooting public, even if they are handloaders.
Factory ammo is loaded with powders not sold at retail. Factories buy powder by the boxcar load, and develop loads with it, to meet desired pressure and velocity specs. Canister grade powders, (the ones sold to handloaders) great care is taken so that each batch is as close to the same as the last one as they can make. So that can of Bullseye you buy today (assuming you can get any) is essentially the same as the can bought 4 years ago, etc.
The powder used by the ammo makers can vary hugely from lot to lot, even with the same "name". They take this into account, and work up loads for each different lot.
So, lot#1234 might use 10.0gr for a certain load and lot#1235 might use 12gr, and lot 1238 might use 8.7gr for the same given load pressure and velocity.
That information has no practical use for us, we can't get the powders they use, And seeing one# lot of ammo used less powder than a different lot# means nothing to us, either, as ALL the lot#s are loaded to the same pressure specs. etc.