You've gotta figure that most new tires have 10/32" tread depth and most BITOGers won't drive on tires that have less than 3/32" tread left. Those little glass shards are either gonna hit the tread, where the rubber is extra thick, or they're gonna basically not even touch the tire if they land in a groove. It's a win/win.
Think of the couple times in your life that you've probably had to pull over on the side of the freeway. You're feeling all that debris on the shoulder like you're the Princess and the Pea. OMGOMGOMG I'm gonna pop a tire and then you're fine.
The rubber industry / tire industry is pretty mature. They do science and stuff.