Because the BIOS has some sort of security that won't allow booting an 'unapproved' OS. These aren't regular X86 machines that you could just slap Windows 10 or something to.
Also, disk swapping in Linux isn't as easy as you might think. For a while I had a system running VectorLinux 6.x with a heavy theme that made it look a lot like those computers in the Jurassic Park movie (Irix, 3D File System Navigator, the console, etc) but just popping that IDE disk out and slapping it into another system was met with a "KERNEL PANIC UNABLE TO MOUNT ROOT FS NOT SYNCING" error. Put it back in the system it came from it booted fine.