Well, you know the problem with a "why" question is that you can keep on asking those questions forever, to the Nth degree, and still never be satisfied. For example, I could tell you that:

1. The catering menu planning software cannot be integrated with the passenger service system reasonably easily by UA's app team, or
2. The flight-specific meal choices are not standardized enough to allow for any kind of consistency or predictable meal choices, or
3. The UA commercial development team doesn't believe it's enough of a differentiator to throw their weight behind pushing for such an idea, or
4. UA's management doesn't view IT improvements as core to its customer service strategy, or
5. UA's web team are too busy keeping the website barely functional for the over-complicated reservations functions as it is already, or
6. ... on and on.

Where should we stop?