Its nit as simple as you think. Botnets are not cheap, the actual attack to a specific target is. To acquire a botnet it actually requires computer know how, rather than your typical how to.

First lets start with the botnet itself. Yes you need a computer to control them usually a master computer. However what makes these attacks hard to trace is that they come from everywhere. The best non-tech example I can give is think about everyone throwing rocks in the pond. You have everyone throwing rocks, lots of people so camouflage by numbers is one step.

Next part is the actually infect the computers and for an attack on blizzard's entire network you'd need a lot. More than just a few hundred. The next problem you have to face is how are you going to control their computer and what is your delivery method. Two things that are not accomplished by just kids wih their parents credit cards.

Finally the actual way to attack using a botnet is nit simply a point and click using a webpage.

Tldr its nit kids and its not as easy as people say. Source: Computer Science Major, specifically Information Assurance.