Because while 95% of the tourneys you enter are going to leave you extremelly frustrated. The 5% where you play your best and get to a final table or even win are worth it. I don't care how great a session you have in a cash game, it doesn't equal the feeling of taking down a 1000 or 2000 person MTT and knowing that for the past 5-8 hours you played better poker than everyone else in the tourney.

After 4-8 tabling cash games or even STTs for a couple hours I feel like I've wasted my time regardless of how well I do.

Win 1 MTT and make that 1 big payday and you'll see what I mean. Poker wouldn't be a major part of my life if it wasn't for MTTs. The strategy and competitivness that they bring just doesn't come out in a cash game.

While I make a good amount playing MTTs, they're the reason I love poker. If it was only cash games I don't think I'd be playing poker as seriosly as I do.