This is a stupid game that Apple plays. Why should I even see the application or be allowed to hit the install button if my device isn’t compatible.
One argument could be to entice users to upgrade, which likely works pretty well. But it isn’t very helpful for those with devices that have no known upgrade path (like my iPad 1). To them, it’s a frustrating game of hit and miss.
By comparison, This is one area where the Google Play store really excels. If your device lacks the requirements, the app just won’t show up for you in the store.