I think there are a couple of reasons. Which is not to say it won't happen (tipjoy rocks), it's just hard.
Payment is about trust. In order use someone to give or collect money on my behalf I have to trust them. Trust for startups is hard.
Secondly, people aren't always interested in small payments. When the cost of a transaction approaches zero it often becomes it.
If there were an arbitrary scale of value, people would more willing to pay $5 for some that gives them 5 value than $0.50 for something that gives them 1 value. They would look for something that gave them 1 value for free.
If the applications being created are very small, e.g. in a few hours or days, as is the case in the content creation industry (for websites), small sums of money work. Websites are running on adsense, 1 dollar for every thousand page views (uses of the site).
As for trust, a third party can run the application and meter use (as in amazon ec2 and payment system), and besides, if a user doesn't pay or a provider doesn't give quality product, you will withdraw your offer from that person pretty quickly. As examples, affiliate systems like commission junction, clickbank, adsense are working.
Payment is about trust. In order use someone to give or collect money on my behalf I have to trust them. Trust for startups is hard.
Secondly, people aren't always interested in small payments. When the cost of a transaction approaches zero it often becomes it.
If there were an arbitrary scale of value, people would more willing to pay $5 for some that gives them 5 value than $0.50 for something that gives them 1 value. They would look for something that gave them 1 value for free.