Aircraft are designed with features to reduce the potential transmission of viruses on board. Cabin air is refreshed every 2-3 minutesmuch more frequently than most other indoor environments. It is also filtered and flows from ceiling to floor, all of which helps to maintain a healthy on-board environment.
This why IATA believes that if a country has removed mandatory mask rules in indoor environments and/or other modes of public transport, then mask-wearing should not be mandatory for air transport.