The reasoning is when you take a very sharp blade and subject it to the abrasive environment of grass and other debris that edge quickly gets worn off, even in seconds in some cases, So within a few minutes you are actually mowing with what would of been technically a properly sharpened blade. The slightly blunt edge is more wear stable than the initial knife edge that is created by sharpening. So you remove material from the blade trying to maintain that sharp edge and then the grass removed even more blade material wearing that sharp edge off and the two combined result is a shorter blade life.
The other issue is the blade is sharpened at the factory prior to painting, and the paint makes the cutting edge look thicker than it really is, and gives the strong impression of a dull blade from the factory.