For some channels, ad revenue isn't nearly enough to make it worth the while to do videos. Merchandise and sponsorships are other revenue streams.
I do understand you point though. For the TWIT network podcasts, Audible is a sponsor. The shows will talk about the sponsor for 3-5 minutes. I don't know if that is a requirement of the sponsor but that is far too much time.
On that note, I did my first referral-based video today. I directed viewers to my website if they want to find out how to purchase the item and I was up front in saying that I would get a referral bonus. To be clear, the referral link is not on YouTube. Only a link to the webpage with the offer and the referral link is on my website.
Since you're curious, 3,500 views so far on the video only resulted in an additional 200 visits to my website. I had 2 sales resulting in $4.89 commission. Since I didn't run an ad on the video, it is not resulting in a net gain yet but maybe viewers are thinking about buying the item and didn't do so yet.