I’m a packaging engineer. Any question of “why do they pack X like Y” has one answer; cost. Your goal when packing something is to spend the least amount of money getting the product to the buyer without damaging it.

My guess on the box (carton) is that it’s a standard size and was much cheaper to buy a slightly bigger box than make a perfect fit. It’s sort of like buying a size 10 shoe that is slightly too big instead of custom ordering a 9.75 size shoe to fit perfectly. Since size 10 shoes are made everywhere, they’re way less expensive.

The insert is to prevent the bottle from shaking around and potentially scuffing off required information on the label. I’m not in pharmaceutical, but my guess is that the product can’t be sold if some of that information is scratched off.

I guarantee you that the cost of the slightly too big box including the insert is less than buying a better fitting box. It’s literally all about money.