They can charge these fees because they are the ones controlling the access to final consumers. They have reached this control by spending billions in building their platform (or even developing the smartphone in the first place) and by keeping the platform secure and simple to use.
For many brick & mortar retail segments, fees can be much higher than 30%. Supermarkets can charge easily 50% of final price as fee (i.e. they buy the product from manufacturer at 50% of final price), and for some categories in some countries it can be much higher still. This then pays for their operations (warehousing, inventory, labour, rent of spaces in attractive areas...).
Yes, consumers pay this fee ultimately, but they are happy to do so in exchange of elimination of the search friction (a vast choice available under one roof). In fact today we have largely moved away from going to the baker for bread, then butcher for meat, the local market for veggies, the hardware store for nails etc. Even dedicated stores (like clothing stores on the High Street) offer choice of sizes, opportunity to try the garments on etc.) and are in areas of high foot traffic that enable searching for the right product by browsing in nearby stores.
Ultimately, though, retail remains a low margin (in % terms) business, a proof that those fees are not that absurd.