It's a pretty big argument, I suppose, which console is better for gaming on. Honestly though, I think the question should be: why aren't we gaming on computers anymore? Well, I can answer that as well as deny my answer, so I am going to do both. The majority doesn't use computers anymore for gaming because it's too expensive. Yes, it's that simple. In order to have a computer that lasts as long as a console you have to put literally thousands of dollars into it. And when that cycle is over, you have to make a new one, pumping just as much money into it. Despite costing thousands more than consoles, I believe that PC gaming is extremely beneficial to a lot of people.
Everyone has computers. Well, practically everyone has computers. But if a person is going to have a computer and console for gaming on, whats the point? When you have a computer that's good enough to play all the new games and such, its capable of a ton of other stuff as well. Stuff that consoles can't do even if they wanted to. Therefore, the benefits of PC gaming are not only a better gaming experience, but a better experience in general simply because there are thousands of new things available to you. And if you are willing to put in a lot of money to begin with, instead of completely building a new system in a few years you can just upgrade the necessary parts over time.
Most of your friends play on consoles? Okay... well keep your console, thats fine. They usually have their exclusives anyway which is a big selling point for them. But PCs have tons of unspoken exclusives as well, and usually get what are considered Xbox exclusives. The best bet for the greatest gaming experience is to get a PS3 and a PC, you get all PS3 exclusives, console exclusive, PC exclusives, and most Xbox exclusives as well.
What it all rolls down to is how much you are willing to put into a computer. If you spend countless hours on a computer anyway, investing in a gaming computer is a great idea. Even if you only spend some time on a computer, but wish you could do more with it, you definitely can with an investment.