> Is it allowed to charge more in storefronts which take these cuts? Why nobody does that?

What? There are plenty of apps charging more when you buy currency/subscription on iOS compared to when you buy from their website, or in some cases Android app. Patreon is an example that made the loudest noise recently, but it’s been a widespread practice for years. That said Apple doesn’t (didn’t?) allow you to tell users that a cheaper option exists elsewhere.