Because once you release your content that can be represented as a computer file, it is bound to show up somewhere on the Internet for free sooner or later. To avoid having to waste your effort, put a pricetag on your work, collect that amount collectively from people wanting it, then follow through and make it available for downloading.
The problem with this model is that people are afraid of getting 'ripped off' and not getting anything for what they paid for.
The solution for that would be an escrow-based version of PayPal that all parties could trust and use:
1) Content creator creates something, puts a pricetag on it and posts it with the escrow service and lets people know it is there.
2) People pay the escrow service for the item until it is 'paid in full'. The escrow service
pays the content producer the full price they set for their item minus a small reasonable handling fee for the escrow service for providing this service.
3) Escrow service releases the content to the masses who paid for it via a one-use-only download URL. Everybody is happy (including the 'freehounds' who got it from somebody who paid then re-uploaded their purchase somewhere else on the internet) -- everybody involved got what they wanted: money or the item being sold.
Unless you buy stuff 'on credit' or pay your utilities 'after the fact', you 'prepay' for all other goods and services you use (like prepay gasoline for your car). Why should 'Interlectual Property' be treated any differently? Time, effort, and money was expended to create it just like an ear of corn, or a washing machine.
The alternative is business as usual -- the same high-stakes cat and mouse game between the 'freehounds' and the content producers wanting to be paid for their content. This is a situation no one can win in the long run....