As one who has been on this and many other forums for a decade, here's my opinion:
1. Most affiliate marketers think SEO is the greatest thing since sliced bread and think that's the only way to advertise. It's not. By the way, really, there has not been anything better since 1928?
2. Making a profit using PPC is hard for the simple reason that it's not your product. If we sell the same product, you as an affiliate, I have a bit of an advantage in that my profit margin may be 50%, even 100% or more. You get a 5% commission.
So say I buy a product for $20 wholesale and sell it for $40, I have $20 to play with for advertising. An affiliate may sell the same thing for $40 and you may get $2, not a lot of wiggle room and a ten-fold difference from what I can do.
3. Affiliate marketers are for the most part cheap. Many don't want to spend a nickel even if it means they'll make more. It's my opinion. Of course, 1 and 2 above have a lot to do with that as well.
4. I'm sure many just don't want to try PPC (maybe for one or more of the reasons above) or because they think it's hard. Well, nobody said it was going to be easy but doing and relying on SEO only is no piece of cake either. If they are satisfied with their methods, have at it, who am I to argue?
5. Because they may think it's relatively simple and easy, a lot have jumped in and got burned. Many reasons for this: not understanding the system, not evolving with it or not following or understnading the rules.
Seems less so now but years ago, it was "don't use Adwords". I think it's pretty much ingrained now and repeated often so AMs don't use it. There's a myth out there that Google doesn't like affiliates which is not true at all.
It's not because getting an account is hard. It's easy and you only need one. That's part of the problem with AMs getting banned for not following rules and trying to get something for free with the coupons. Or cloaking. You really think you can fool these guys? As thebomb said, successful people don't need that shit. It's also a hell of a way to try to run a business, which most AMs I know say is what they do. It's not a business unless that's all you do and earn a living doing it. Few fall in that category.