You've got two choices as far as bootloader design goes. You can put the bootloader in a well-known location (such as in the first 512 bytes of a drive), or you

You've got two choices as far as bootloader design goes. You can put the bootloader in a well-known location (such as in the first 512 bytes of a drive), or you can tell the firmware where to look. UEFI actually supports both, but the problem with using a well-defined location is that you inevitably have conflicts between different OSes wanting to use the same location. Boot variables are well-defined and work just fine, except in the small number of cases of vendors who have implemented something that's not actually UEFI.