This.

Not just the mining aspect as has been pointed out above, but there is a PITA factor at doing stuff on an app that can be "easier" on a computer. The more complicated the transaction, the bigger the difference. Imagine buying a 2-pax airline ticket from a brand you've never used before, on a smartphone. Certainly doable, but all those fields to complete, and complete accurately with no typos. By getting you to build a profile in their system and use their app, all of a sudden their brand is much easier to use than a competitor's.

In my case, with AS or UA, I have their apps installed, I'm all profiled, they have my names, my common travel companions, KTNs, frequent credit cards, it's quite simple to book. And to some extent the apps are not as fat as the website (cough, UA, cough), it's actually faster to book on the app than the site since I'm all profiled up. I still prefer the full site for complicated itineraries since I'm picky on options. But for quick and dirty nonstops, the app is faster and easier.

The PITA factor of using a competitor's system is the barrier to using the competitor.