The OP report was very much at odds with my experience on UA availability.

I checked a number of dates in July for two passengers nonstop ORD-SEA.

Among the 10 or so dates I checked, I did get availability every day.

But, the only flight on which Saver seats were available was the 10:25 pm departure, arriving SEA at 12:42 am (or nearly 3 am Chicago/biological time).

Yes, UA does have the availability on that flight. I'm not surprised. How many paying passengers would opt for a flight with those timings?