Not saying it's the right decision, but probably because they don't want the liability if a rooted device allows some 3rd party app to hack theirs. Airlines can be somewhat like a small bank in this respect, as being logged in gives you access to things like accumulated loyalty points, not-flown-tickets that can be exchanged/refunded, past flight credit from earlier refunds that were eligible for credit but not a cash refund, etc.

Also, they typically store more personal info than, say, an e-commerce application because some of it is needed for travel. Things like date of birth, known traveler numbers, names/dob/etc of family members, passport numbers, etc.

Likely a DRM requirement so you can’t pirate their movies