Why can't a shared flight carry more than six people?

Because it is one of the conditions that make a shared flight legal. Cost-sharing is limited to a maximum of six occupants including the pilot, meaning five passengers. Beyond that, the flight falls outside the scope of the exception.

A limit set by the law, not by Volara

This rule comes from the European regulation that governs cost-sharing (Regulation (EU) 965/2012). It is not a commercial choice on our part: it is the very condition that permits cost-sharing. Six occupants, including the pilot, not one more.

Beyond that, it is no longer sharing

A flight that carries more people is no longer a sharing of costs between occupants: it becomes air transport, reserved for operators holding an AOC. That is why Volara does not accept a listing that exceeds this limit.