I think this makes sense, because in most cases 3 different text ads are competing against a single image ad. No matter how strong the image, it may only draw a few more clicks than the best of the text ads available. When competing against all 3 text ads, it doesn't win.
In other words, a single large image promoting a single advertiser, with a single offer, just can't outperform 3 smaller text ads from 3 different advertisers with 3 different offers.
But, at least in some niches, the right image can draw a lot of attention, or trigger a powerful emotional response. Thus, the single image can potentially offer a higher CTR, or higher payment per click, or both.
In any event, none of this is a reason to reject DFP. The better reason for ignoring DFP is the complexity/learning curve.