Also, this issue is not limited to airports - there's lots and lots of integration to be done.
Say, if I want to take a bus in Bergen (Norway's #2 town, or, if you ask the locals, #1) - you need one app to find out which bus(es) to take, then another to buy a ticket. If you want to buy a ticket for the airport coach? Oh, you'll need another app for that. Actually, you'll need two, as there are two coach companies.
And, getting to Oslo, (#1, or, if you ask the people in Bergen - #2) you'll need another app if you want to take the shuttle train into town, then yet another for getting tickets once you transit to trams/buses in the city centre.
&c. If public transport companies could agree on a way of sharing payments, the market is more than ripe for unified apps for route planning and ticketing, at least on a national level. (Expecting standardization to cross borders is probably a bit of a stretch - and, unless you happen to live in, say, Liechtenstein with half a dozen countries within walking distance (yes, I exaggerate), not much of a concern, really.