They're big corporations who look at numbers rather than the people behind them. If your band started a buzz by farting and belching, a major label would release a CD of you if they thought they could make any money. You've probably heard the saying that banks don't like to give out loans to people who really need money. Those with good credit get good rates, while those that don't have good credit have all kinds of extra charges and hoops to jump through. The same could be true for major labels and record deals.
Labels love to pick up acts that already have a good foundation in place.
It's less work for them as well as less chance of losing money. You have to show the majors that you don't need them in order to get their attention and get a good deal. Dave Matthews , KC and the Sunshine Band , Def Leppard , Ani DiFranco , and Hootie have all put out their own releases and done very well on their own without major label help. Those who have decided to move on to major labels have gotten great deals that have made them megastars. Those who haven't signed on with a major are still making a very good living.