Why do airplanes land sideways?

Answer 1

They don't actually land sideways, but will approach sideways depending on the amount of cross-wind. Aircraft turn into the wind, like a weathervane, so if coming in to land on a east to west runway, and the wind is from the southwest, then the aircraft will point southwest during descent. Just before the tyres contact the runway, the pilot will give it a bootful of right rudder, to align the wheels and runway. Once the tyres are in contact, the aircraft will keep reasonably straight along the runway, the pilot helping with brakes and rudder as required.

Answer 2

Airplanes do not typically land sideways. However, in certain situations such as strong crosswinds, pilots may perform a maneuver called a "crab landing" where the aircraft approaches the runway at an angle to compensate for the wind. As the airplane touches down, the pilot may then align it with the runway using the rudder to prevent drifting off course.