Several reasons:

- payment either requires using private cryptocurrencies, that most people don't use, or exposes identity to operators - vouchers make purchase and payment unlinkable

- direct payments create risks of operator default - we want to build zero-trust model

- no effective way to codify revenue-sharing agreement - again, risk of defaults, not honouring agreement, etc.

All these problems are solved with smart contracts, and we don't see any simpler solution.

The question is similar to "why SimpleX network needs private message routing?" To protect users privacy and security from their contacts. As the number of operators grow, it's important to protect users from operators.