It's simply a matter of math. Unless you have an absurd amount of computing power (way more than a few GPU's) you're just never going to find a block in a reasonable amount of time, even if the difficulty were to never change. But it is changing, and going up rapidly. It's entirely possible you would never find a block at all by solo mining when you factor in the difficulty increases. On the other hand, with a pool you're pretty much guaranteed to get something for even a modest amount of effort.

I'm pool mining with a mere 500 MH/sec and I'm getting about 0.18 BTC per week. That's not much, granted, and a lot of people will tell me how I'm wasting my time because I should just buy BTC, but I'm not mining for quick profits. I'm doing so because I like the idea of mining, I wanted to learn more about it, and I thought that it was a better way of acquiring BTC than dealing with the exchanges. I'll probably hold onto my meager earnings long-term and see where this whole Bitcoin thing goes. And I'll probably switch my GPU's to some alternate cryptocurrency when and if my BFL order ever arrives.