It can't be easy to run a profitable airline when the vast majority of the passengers look for the lowest fare. If I were in UA's shoes, I would probably schedule the flights the same way. Each flight has slack time built in. You often don't end up using the slack time, so you might as well make it do double duty, e.g., by shortening the turnaround time. Even if the flight departs late, you can often still arrive on time.

Sometimes it works, sometimes it doesn't. Just yesterday I was on a flight where the inbound aircraft was scheduled to arrive six minutes before my flight was scheduled to start boarding. It actually arrived nine minutes early, and we ended up boarding on time. But I agree that it's not uncommon for the boarding door to close about five minutes late. If I can fly 1,000 miles for under $150, I'm not going to worry about a few minutes.