Oh they have their hitches.
But besides that, the point was that matrix and fileplanet are not sharing that system so others can do it. And writing one from scratch is a big headache. Well, fileplanet is in a way but to offer things thru them cuts into the programmers profits.
As to the method you meantion, thats not very workable on a large scale. Downloads still work for things like demos where broken downloads dont matter, and snagging multiple copies doesnt matter, complaints of non-delivery that can be abused, and many other factors.
I know, I see it all the time in the gaming newsgroups also. It seems like there are no headaches and anyone could write one. Therefore the only reason its not done by everyone must be a conspiracy because it would be so much better for "everyone".