Quote; "My bet is that it's because the decimal system is our friend."
This.
Other than bulk items, packaging for most stuff comes in round numbers. And even if you buy in bulk, the bulk containers are usually set up in round numbers. And as noted, uniform packaging allows for placing items on a retail shelf in a manner that has a pleasing appearance, as well as being efficient in space savings.
Lastly, since we are talking ammo,,, lets say you have a 9mm. Is it a semi-auto, or a revolver? Does it carry 7, 8 or such in a single stack magazine,,, or is it a double stack magazine carrying 15, 17 etc? Or is it a revolver with a 5 shot cylinder or a 6 shot cylinder. Too many variables in the end uses,,, so there is no way any manufacturer can package a single product, much less a lot of different products to "fit" all the variables.
Bulk, loose ammo purchases (like the way we used to buy nails & screws,) could be a disaster as well. I buy my screws & such in loose bulk, (Yes, I have a few hardware stores that still do that,) and I often get an odd size mixed in. Can we say "disaster" if a hot 44mag was accidentally mixed in with 45 Colt ammo & put in an old gun built for BP ammo?
Not a good idea.
As noted,,, just spend a few extra moments at the range & fire off those odd rounds at the end of a box.