Perhaps the style of online advertising diminishes it's value. An awful lot of advertising just presents the product, expecting the viewer to click on it. If the viewer follows up on the ad at a later date by visiting the vendor's website directly, there is less revenue earned by the advertiser. Couple that with people simply refusing to click on the ad (many people have been trained to not click on suspicious links, which a lot of advertising qualifies as; some don't as a matter of principle) and the perceived value of online advertising is even less.

Then again, maybe online advertising doesn't work. If someone is unwilling to pay for the sites they visit, who says that they are willing to pay for the products advertised.