Why is the winner still awarded $1 million U.S.dollars?

I think the simple answer is they don't need to increase the purse to get contestants. They are clearly getting a broad enough applicant pool to select the contestants they want. Plus you should consider that after 30+ seasons there is an increased fame factor, so people are drawn by money AND fame. Fame can be a powerful motivator. There was a show in the UK called Nothing but the Truth whose format has been copied in other countries that had its Greek version taken off the air "for encouraging contestants to "humiliate themselves for a reward", with "no regard for the players' decency or the effects on the social lives of their families"." Survivor clearly isn't in that same class but it does go to show that people will do crazy things for less money than offered by Survivor on the TV.

Also I think it is important to note that $500k net isn't peanuts. There are shows like Jeopardy! that have been on the air for ages, with lower payouts, and Jeopardy! hasn't bumped payouts since 2001. People go on Jeopardy! mostly for factors besides money, although I'm pretty sure the money helps. I think it is reasonable to say contestants on Survivor compete for more than just the paycheck.