Quite a few of the themes came packaged as ThemePark and Magnifique plugins, while others had DMG installers (so just like installing an app), and others need manual installation.
That is what I would expect, generally. To have them bundled together in place saves on having to hunt the eight corners of the internet to find them (which, generally, is getting tougher as more time passes on and old stuff falls away from even archive-dot-org). And also, to have them all together, for posterity, is liable to be helpful to more people as time passes.