Depending on your socioeconomic status, the $12/year for a .com is either cheap or expensive, and while I hesitate to claim I'm disgusting wealthy (because I'm not) I, (I'm not the OP) at least rich enough to spend $12/year amongst other life's expenses. Having a custom domain name for a thing is a way of driving people to the domain, instead of focusing on yourself. It feels less self-aggrandizing to point at a project on its own domain name, rather than say hey, look at my blog which is branded with my name. What's important is the project, not me.