I guess it's not surprising because Google's AdSense program does compete directly with affiliate programs.
It also seems many feel that Google AdSense is the poor second cousin to affiliate programs. I would say that's true for many but for others the Google AdSense program is much more like a rich old uncle.
It's also true that many do NOT making any money with Google AdSense, but the same is also true with affiliate programs. In either case it is all in the execution.
Affiliate programs are great. I have been a full-time affiliate since 1999. I love affiliate programs, but Google AdSense too!
The thing I have found about AdSense is that if you want to make real money with them you need to be "all in" so to speak. And by "all in" I mean the page design needs to be very AdSense friendly. Turn off all the distractions. Eliminate 90% of what you have displaying in the column of a typical site. Turn-off the comments. In other words eliminate all of the distractions and unnecessary links on the page.
You also need to pay attention to your analtyics. You need to know which pages on your site are earning you a $1 or $2 for each click ...and then work hard to get those pages ranking in the natural results, especially Google.
A page that has a 20% click-thru rate that has a hundred visitors a day = $20 a day or about $600 a month and that's just from one page. Put 4 pages like that together and you would be earning $2400 a month!
Google recently reported that they pay a whopping 68% of the revenue collected from advertisers to publishers for ads that appear on their sites ..and rumor has it that Google does collect a fair bit of revenue from their advertisers.
I love affiliate programs and Google AdSense too!
PS - What's been your experience with Google AdSense?