@Thomas:
First:
1. No regular Amiga has a CD/DVD player per default
2. Even if a CD/DVD is added, especially to the more popular models like the A1200, it is not trivial for many users how to make the computer read the CD/DVD.
If they had put the real OS stuff on disks and optionally stored all the unecessary crap which can be found on OS3.5/3.9 CD, on a separate CD labeled "CRAP", they would not have needed more than about 10 disks.
Sure, that is a handful of disks, but not unreasonably many and it would have made the OS3.5/3.9 installation much more user friendly.
/Patrik