This 30% number is accurate but hardly all tax.
First, companies attempting to compete with Steam have found it costs 12% to 18% to just run a digital store, without all the app review, customer discovery & acquisition, hosting and delivery, and ecosystem integration Apple and Steam are doing, much less human support.
Second, just a credit card transaction can cost 2.5% - 5% depending on the customer behavior. On a percentage take versus value provided, that seems much worse than the store cost, and it’s a huge chunk of the store cost.
Third, Apple lowers the recurring publishing, hosting, transaction, and support fee in year 2+ to 15%, still including the credit card processing.
It’d be incredibly difficult to operate a trusted digital app sales ecosystem (with human app review and support, refunds, etc.) at scale with less overhead.