I think this is just the same problem of micropayments on the web: nobody’s “solved” it, so free is just the most convenient option by far.
Amazon had a service that was built mainly with micropayments in mind. It never took off.
"Amazon FPS’s aggregation feature lets developers track and aggregate micro-payments into a single payment transaction, saving on transaction processing costs and avoiding having to build complex ledger functionality into their own applications. Using the aggregation functionality coupled with the lower Amazon FPS fees, developers can now pursue micro-payments businesses that previously have been cost-prohibitive."
Edit: Better Source and a quote