I'm not saying that every single person stops reading at the first answer, just that people drop off over the page, and in my personal experience (and talking with others), it is common to miss that there are higher ranked answers below the accepted one.

What's the value in pinning the OP's selected answer to the top, even if it has -100 votes? Or even if it 100 votes, but the second has 900? Shouldn't the overwhelming will of the community take precedence over OP's selected answer, who may not have bothered to review the answers for years?

Also, I'm sure the claim isn't that everybody stops reading after the first answer, but that the first answer does get a lot more reads than answers further down. I bet it would be possible to prove this with the right analytics.