Actually, it's more likely because 80+% of Google users couldn't (and probably still can't) tell the difference between the advertisements and the results:
http://www.pewinternet.org/Reports/2005/Search-Engine-Users....
http://www.pewinternet.org/Reports/2005/Search-Engine-Users....
I have to say I kind of feel cheated by that trickery, every time, and it is one of the reasons (though not the most important one) that I use Google much less recently (in favour of DDG).
[1] no I'm not particularly happy about this screen, the rest of the machine is great but if I had known it would be this bad I might have picked a different model (I got an Asus EEE 1215B)
I've may have seen far more of such pages. But obviously i can't tell since they're near-impossible to spot the Ads.
The trend is keeping things "by the book" from search engines point of view, And manipulate humans in subtle ways by confusing contents and ads.