Couple of reason.
Some like to earn their pro cards to help with their personal business.
After you earn your pro card, it takes the fun out of competing. ( I have heard this from the mouth of pro's)
Money-the amount you put in, is nothing to what you have a chance of winning. Just doesnt add up for alot people.
Just because they got a pro card, doesnt mean they have a chance on a pro stage.