MorphOS never had "retail" versions. There have been physical special editions at few events as prizes etc, but not sold versions.
There isn't that much need for physical copy either. It would get obsolete soon. You just can download the latest version for free always. And if you want to register it, you get the keyfile which works with all versions.
And as MorphOS works on many different setups, they want to make sure that you don't buy it for a system where it wouldn't work. So, you have to do the registration from the OS itself and then everybody can be sure that it'll work.