Ronin,
That would be impossible for the airlines to build in seasonal "cushions" for the weather. With landing and departure slots so heavily regulated, it's not like CO, UA, or whomever can build in an additional two hours into the flight time in the winter let's say, and show up that much earlier on a clear day and expect to shoo right into LGA, EWR, or ORD.

Besides, who'll want to book a flight only to learn the flight time'll be 4 hrs 37 mins. LGA-ORD: "Why the long flight time? Well in case O'Hare gets socked in due to snow. Look on the bright side, you may arrive early!"

WX is a fact of life that we cannot control.