Plus nobody pays for Apps on other platforms. Developers who release on iOS and android see 10-100 times the revenue on iOS. Partly this is piracy, but a lot of it is culture.
More than that, even on iOS the paid App model is going away and free apps with in-app-purchase or other premium models are what's succeeding. (not watch this get downvoted.)
However my anecdotal evidence is that having paid for apps on one platform isn't a barrier to switching for a lot of people. It seems to be an afterthought, a realisation only when they come to download (and pay) for the app again. By that point its too late.