Apps and websites will always have maximum session lengths (i.e. length of time you are logged in) for performance and security reasons. Especially websites which manage high-value products or data. Being permanently logged in would be like leaving your front door open when you're away from home.
However BA's approach to making it seem like you haven't logged out, especially in the app, really isn't seamless or graceful at all. Even the fact that the integration with the booking engine is awful, it should be possible to re-establish the session with BAEC immediately on the app when accessing it, making it seem like you never logged out!