Incidentally, if HN allowed it, I would have linked the above text to:
https://en.wikipedia.org/wiki/Rickrolling
...in order to be a little meta.
The middleman isn't checking availability every time because there's a cost to ask the carrier. And if you ask too much they will rate limit or deny you which means you get no availability.
The middleman is a price aggregation person, and tries to find rates and routes using all the rules the carriers specify.
Sure you can buy tickets directly from AA or UA. But what if you want to fly a connection that has both. Do you think AA wants to sell seats on someone's else plane?
Talk about fake it ‘til you make it.
Worked out for me once during a trip to NYC where my hotel was full, but they sent me to a nicer hotel right over Times Square.
I don't think keeping unbooked rooms "just in case" is economical so if this sort of thing happens you may well need to be moved out to a nearby hotel at their expense.