Because it’s a pain in the arse to design, manufacture and build a specialist device just for use in your stores.
I’m sure Apple could do everything that box does and more. But why bother designing, building and manufacturing your own specialist device when someone else already sells a perfectly good tool that does the job.
Don’t forget this is for use in a retail store by people who will have been given 5mins training on how to use the device. You want something that just requires a person to plug two phones in and hit a big “go” button. And it needs to work 99% of the time with zero messing around.
They built specialized tools to update iOS through the cardboard box without opening it before it goes on sale. I’m sure they can build something with a big “go” button if it’s important.