Majority of Linux distributions available to install these days are distributed in the form of single ISO file which conforms to both El Torito standard and contains a bootloader so that ISO image can be copied using dd both to the CD and an USB stick and be understood by BIOS/EFI. However, there are systems such as NOOBS for Raspberry Pi and others mainly for embedded devices which are not distributed as an ISO file but as a set of files that can be copied to an external storage such as SD card directly using a normal cp command right away. For example, the following files are distributed in NOOBS:BUILD-DATAINSTRUCTIONS-README.txtRECOVERY_FILES_DO_NOT_EDITbootcode.bindefaultsosrecovery.cmdlinerecovery.elfrecovery.imgrecovery.rfsrecovery7.imgriscos-boot.binThis looks like a more convenient way for an end user because it does not require usage of dd and therefore does not result in wiping all data on an external storage. Why are not all Linux distributions distributed as such instead of ISO file? A set of files can be archived so it's not a problem distributing a single file vs many files.
Distribute Linux system as ISO vs a set of files
system installation;iso;software distribution