Just like any reseller restricts product access. And? And no they are not restricting app access to other OS's, nor could they. If you want to make your app iOS, Android, and Windows compatible, no one can stop you. Happens every day.Apple actually prevents alternative app stores from becoming a potentially viable business by restring app access on iOS and iPadOS. They are stifling app access competition on major (as part of duopolies with Android) mobile and tablet operating systems.
Being a business elevates the importance of profits and profit margins. That's the point of having a business. Making less money doesn't translate into reducing costs of your product. In fact, running a leaner company incentivizes the business to not reduce costs or run sales. They can't afford a dip in income generation.Apple has the overhead they do because it’s a global company employing over 150,000 people, is involved with various types of products, etc. Being a public company can also elevate the importance of profits and profit margins. Smaller, leaner companies don't necessarily expect/need the profit margins Apple seeks and are also able to provide competitive options and ideas that Apple may have chosen not of felt no need to because they’re been able to restrict app access competition on iOS and iPadOS.
Oh and the App Store is like a sub-entity within Apple and has it's own financials. The overhead for App Store in mostly generated from storage and distribution resources that an independent app dev would be responsible for if they sold their app on their own. App Store also acts as Tier 1 tech support for most apps as well. So if my weather app bugs out, I contact Apple first. That's a very expensive resource that the app dev is going to have to front the cost of independently. Again, your prices aren't going to drop and they might actually increase because the app devs are going to have to provide storage, distribution, tech support, etc. Their overhead is going to balloon and that's going to translate directly to app pricing.