A app store offers reliable hosting of the game binaries and provides a versioning and upgrade mechanism on a platform with billions of user accounts.
Keeps the user's OS and device drivers up to date, maintains the OS and develops the hardware that it runs on.
Maintains the developer tools, compilers, documentation, so that your binary can run on the device's OS.
These things are expensive. I don't know how much exactly it would cost a company like Roblox to develop and maintain all the necessary tools and services provided by the app stores in order to run on all those devices.
Probably a lot. In fact, if they could do it, they would be Apple or Google :).
Whether it's a lot more than it should be, I don't know, but it helps to keep in mind that the app store cut is for a lot more than just payment processing.