The policy (# of miles needed) is based on regions and not on miles -- that's just the way its. To add to your example you can UG from coast to coast in the US for 15K miles and some flights are 6 hours or more.

There needs to be some system and United chose a region based approach. Any system will have inequalities.

A time based system with breakpoints say a 3 hours, 6 hours, ... would create a problem where flights in one direction between two cities might be below a threshold point and above threshold in the other direction. Also, do you measure times from gate to gate or take-off to landing?

A mileage based system be the "most equable" -- similar to the e500 approach.

But United and the other airlines are going to be more driven by what their competition does more so than what is equable.