Why is the fifth wheel trailer hitch called the fifth wheel?

Answer:

Answer

not sure, but probably due to spare wheel(fifth wheel) used to be kept in pickup bed, which is where fifth wheel hitch goes.

Not quite

In the days of horse-drawn wagons, the fifth wheel was a wheel placed horizontally over the front axle of a large wagon to facilitate turning. Basically, it allowed the front axle to pivot.

Today, the coupler of a tractor-trailer truck is called the fifth wheel, mostly because it's round (somewhat) and is vaguely reminiscent of the wagon's fifth wheel (which much more closely resembled the wagon's other four wheels).