While I don't disagree with you, the findings of studies are hardly factual. Scientists and doctors understand this, but the media that presents the findings to the public (via newscasts, etc.) over-exaggerates the validity of the conclusions. Not to mention most of our modern medical knowledge was only discovered in the past century.
In other words, just because 7-8 hours is suggested does not mean it is necessary.
If we were to put this in terms of formal logic. Let's assume the following: if you get at least 7 hours sleep, you will feel well-rested. If we negate the first part (i.e. if you get less than 7 hours sleep), that does not imply you will not feel well-rested.