Why do tires become less sticky with time? (not wear and use, but simply over time)

I have 2014 Michelin Pilot Super Sports and I've been told multiple times that the reason I can't launch or have poor grip is that the tires are old. Even back when they only had 15k miles on them (when I first bought my car). There is plenty of tread left! via /r/cars http://ift.tt/2torFi9