I don't think it is a problem that is harder on mobile than web, it is just that up till now for mobile we have only focused on downloads: downloads is the main metric for iTunes Store and Google Play to rank apps and to get into a top 25.

I totally agree that this is a stupid metric and we should move to monthly active users, both for general measurement of success as well as ranking in app stores. Getting a lot of one time visitors on your web site is not hard, monthly recurring visitors is. No different on mobile, let's start dealing with the challenge of engagement on mobile as well.