Most Basic, Newbie Question

I haven’t even started out yet - just have a basic question.

I have read that affiliates can get up to 20% commission per sale. Compared to getting pennies for PPC, that could be a huge difference.

So my basic question is, why would people not primarily go with the commission route over PPC?

I see from reading here a little some folks are using PPC to get traffic for their affiliate sites. That seems to make sense - does it pay off? Still though, just browsing around the web I seem to see a ton more pure PPC than affiliate marketing - or is that just a misperception?

Thank you