It depends on the engine but indy studios are frequently able to port to all of the major platforms: PC, Xbox, PlayStation, and Switch.

The exclusive addressable market for each is definitely big enough to outweigh porting costs.

A good first step would be to ban exclusivity agreements, rather than mandate cross platform availability and to fine both parties heavily for suspected foul play.

Exclusivity agreements are a great example of something that is directly harmful to consumers. It removes choice, increases cost, encourages electronic waste, and leads to an inferior experience.