I'm not an e-sports fan, but there are plenty more monotonous jobs that people do for a living other than playing videogames. Is it silly? Maybe. Would I enjoy playing games for so long every day? Not really. Compared to watching "reality" contestants humiliate themselves to make a "brand name" for themselves, this is perfectly respectable.
As the others have replied, prize winnings, team salary (teams generate money from sponsorship deals and selling merch). Also TLO (like many pro's) streams his SC2 sessions via twitch.tv and generates ad revenue.