By providing some small value-add products that are not possible easy for individual instructors to provide, but customers like to see. E.g. nicer scheduling of lessons or a way to share sheet music with the customers?
With platforms like these the choice isn't really up to the supplier, but to the consumer. The consumers come to expect those value-adds and so the suppliers have to switch to the platform (where the revenue share is opaque to the consumer) to get gigs.
The 20% cut also does sound rather extreme, but if you build a very good product where you can end up charging even more than the usual price, the 20% isn't taken away completely from the supplier, and might end up more like a 10% actual cut.
With platforms like these the choice isn't really up to the supplier, but to the consumer.
Once the platform has a significant number of suppliers, sure. If the platform is new then attracting suppliers is the number one problem to solve, and that's really hard if it's too expensive.