Because all but the sharpest stuff rolls off - it tends not to stick. Soft squishy tires, by comparison, tend to grind the stuff in.
This is actually something that has held in cycling for a long time. I understand you are skeptical; part of me still is. But I find that when I put the air to 'em, I get fewer flats.
As for comfort, well, I ride a road bike for both commuting and sport. I don't really expect comfort in terms of a plush or luxurious ride.
Hard tires mean fast and smooth, to me. Comfort is for La-Z-Boy chairs.