It is, when it was not designed to do so in the first place. Android is built differently.
Apple also never stopped you from creating a WebApp. You can always bypass the store. The first iPhone allowed developers (and still do) to create WebApps. Microsoft is doing this exact thing right now with the xbox gaming cloud web app. There are clear benefits to having a native app (hence the creation of the AppStore). But, if your end game is to make all the money and pay Apple nothing. You can do so with a WebAPP. But, developers don't want to do that. Since well, they have to host the data they provide and manage payment systems across the globe and you know, deal with happy and or unhappy customers. Advertise it, pay money for that, etc. etc. etc.
So, the heck with that. Let's use Apple/Google Appstore for all that and just collect all the money at the end.