Getting to the point faster: It is because of Jet Streams!
Now that you know this, read on. You are 3 minutes of reading away from knowing something new.
If you are an aviation enthusiast or a frequent traveler (or even a not-so-frequent-but-ever-so-inquisitive traveler), you would have noticed this one thing: Airplanes fly faster when going East as compared to the going West. And the speed differential is not 30-40 knot, but reaching way more than 250 knots (460 kph) in many cases!
So while the United Airlines Boeing 777 is flying at 652 knots at cruising altitude (height where a plane stays for most of its flight time), the Singapore Airlines Boeing 777 is at 372 knots at cruising altitude.
One more example can be the Korean Air Boeing 777 in Image 3 flying at 542 knots and the Hawaiian Airlines Airbus A330 flying at 445 knots.
This is counterintuitive to an extent as the Earth is also spinning from West to East. As per common sense, you should take longer to fly in the same direction as the Earth’s spin because as you move towards your destination, your destination is also moving away from you! (Actually, Earth’s rotation has minimal to no effect of airplane flight times, so this ‘common sense’ is a flawed common sense)
Then what causes planes to travel faster when flying east?
One-word answer: Jet streams (Sorry for writing two words in a One-word answer)
Jet streams are high-speed winds traveling West to East at high altitudes which typically coincide with the cruising altitudes of normal commercial jets. And these winds can be 70-250 knots or 125-400 miles per hour.
Pilots tend to ‘ride’ on these winds and attain higher speed relative to the ground. What’s the benefit for the airlines? Less fuel (which is VERY important!). Also, High speed = Less time!
So on your next New York to London flight, thank god for Jet streams. And curse them on the way back.