Why do gliders have long wings instead of short wings?

Simply speaking it is the most efficient design for a glider. A long wing span (tip to tip) and a short chord (front to back) will produce more lift and less induced drag (drag created when lift is created) than a shorter wider wing. The shape of a wing is dictated by the application. An aerobatic airplane has a short wingspan so it can roll quickly, a glider will roll much more slowly (like spinning on a computer chair and putting your arms out and in)

There are many other design elements to a wing that change its characteristics - camber, sweep, wash, dihedral or anhedral, taper, flex, vortex generators, fences, winglets and other tip devices are a few. Each has its own purpose and changes how the wing behaves.