I can see the point in road bikes having less clearance than dedicated touring bikes and certainly much less than mountain bikes. But it seems like many current designs go out of their way to minimize the clearance even when that isn't necessary for a shortened wheelbase and use of short-reach brakes. Having seatstays curve in so they just barely clear 23mm tires adds no functional advantage and just makes the bike less versatile if the owner wants to use it on bumpier roads or wants the extra cushioning of 25 or 28mm tires.

Fairly recently we had a rider show up with a new bike for a club ride and noticed that the rear tire was rubbing slightly. It had the original 23mm tires, but there was well under a mm of clearance to either seatstay. Took us a few minutes of fiddling with the exact positioning of the wheel in the dropouts to find a spot without any rubbing. And that was with a wheel that was properly trued and a pretty narrow tire. Why cut it so close that there isn't margin for any slight lack of trueness of the wheel or use of a tire that happens to be a bit wider than average?