Because the jet stream and prevailing winds in this country tend to run from west to east. So flights from New York to California have a headwind while flights from California to New York have a tail wind.
Imagine a jet traveling at a true airspeed of 400 knots. With a 100 knot tailwind, it will be traveling across the ground at 500 knots. An identical flight going close to that same altitude but in the opposite direction would only be making 300 knots across the ground. Now do you see why one is less than the other?
