It's market segmentation. Most of the traveling public is cheapskates who will buy the lowest fare they see, no matter the consequences. This is an option for those who aren't purely driven by budget to pay for a better experience. I guarantee you that if United increased fares to hire enough staff to provide the same level of service you get in the lounge (with almost no line to talk to someone even in event of a major disruption), most people would switch to another airline.