Because there are price sensitive users and many services would prefer to share those 30% with them instead of losing the business. Because even if the developer gets the money, they will invest them in their product instead of them going to Apple. Because even if they don't invest them, more money on the market will incentivize more developers who will create competition. Because even nothing changes, this means more money around the world instead of being concentrated in the Apple shareholders who will use them to create even more locked gardens.