The reason is very simple, you only get to call ExitBootServices() once (absent hacks that hook the function). If you want to be able to do anything prior to c

The reason is very simple, you only get to call ExitBootServices() once (absent hacks that hook the function). If you want to be able to do anything prior to calling ExitBootServices(), such as choose what EFI application you want to use and options you want to pass it, you need a service built to provide you that interface which itself does not call ExitBootServices(). The name of that service is the bootloader.