I am always left to wonder that from all of the people that moan about these fees, how many would support their elimination in favor of a $10 increase each way? From what I understand through working with people at United over the years in our joint venture, the domestic airline industry in the US is dominated by whomever can offer the lowest fare. By introducing these fees to the lowest reservation classes, airlines avoided a fare increase which thus allowed them more customers. People don't think about it when booking their tickets and then just decide to get pissed when they show up at the airport. There is always the option of booking higher fare classes which have fewer restrictions and fees, but everyone always wants the cheaper number on the booking screen.
The US domestic airline industry, especially with legacy carriers that control the market, is not really a big money maker and is typically used primarily to feed intercontinental flights which attract higher-yield customers. There's no point in building costs into basic airfare to add services which many customers do not use, especially when the competition keeps their base fares low below extra fees. Delta doesn't really care if mommy, daddy, Jimmy, and Sammy fly to Disney World with Southwest as they would only bring in very little revenue (and in many cases lose the airline money). These airlines really are not around to carry people from Chicago to Orlando, but rather across the continent or over an ocean. Low-yield passengers typically don't hold any degree of loyalty to an airline, thus there is no large incentive to provide them with freebies which could otherwise be used to supplement the small revenue stream provided by a discount-economy fare. Everything collectively under full Y tickets exists only to make another 1-2% in net revenue on the route. Frequent, full fare economy, business class, and international flyers are the ones that bring in the money and are thus the ones that matter to management in the US; they can have their bags, premier customer service access, etc. Everyone else is there to be supplemental, and without the bag fees their collective presence would most often be destructive.
You get what you pay for and can't have both the cheapest fare alongside other services that many flyers don't need/want. If premium/high-fare passengers were not there to pay for and subsidize the vast majority of a flight, those that travel on cheap tickets would be often paying 3-6x more to fly, but hey you'd all have free checked luggage. But you get to pay less because of lovely price and service discrimination practices. And if you still decide to pay the cheaper fare, I'm sure that UPS or FedEx would be more than pleased to move your luggage at such speed for far greater sums of money.