Why do they always remind people to place more blocks or blocks with more ads on a site?
This does not make sense at all. Only the first ad is the highest paying ad. Do the math. If you place more ads or more blocks you will always lose money.
Your math is correct from your viewpoint.
But look at it from the viewpoint of Google: they have crappy ads to run somewhere - the advertiser is waving dollars at them to do it. They will sell out the best paying ads, but those lower ones need some extra help. So instead of all publishers in a certain niche just showing the the top 2/ top 4 of the best paying ads or so, imagine them all showing the 12 or so of the ads. The odds of a user (accidentally) clicking on some ad increases, and some of those clicks land on the lower grade ads, selling some of them too and encouraging the advertisers to stick around while at the same time not hurting the clicks and interests in their top advertisers as tehri budgets are used up by the end of the day anyway.
So more ads per page is in Google's financial interest, not necessarily in that of the publisher.
Long term the whole story changes even more: merely having ads hurts us as publishers (less good natural incoming links, shunning by some, ...) more ads is even much worse to our reputation.
That's in the end why I run only a single leaderboard on all pages. Even if the page is long and full of text. Anybody actually reading it is interested and I don't want them to run away for a crappy ad. I want to sell them some affiliate books and read more; or sign up for an account in the forum, or ...
I see adsense clicks as lost leads where I do get a small kickback for but I'd rather not lose really interested users as they're for more valuable to keep than hand off for mere pennies.
Luckily the lack of target matcting due to the interest based ads actually helps in this: those clicking their interest based ads are not my core target audience.