Because pi is an irrational number that has an infinite amount of digits
Any mathematician will use the number pi (and its symbol) sooner or later - it is a number used extensively in many different areas of mathematics.
The first time the symbol Pi was first used for Pi was in ancient Greece in their numbers. The symbol "π" was number 80 in Greece.
Since pi is an irrational number it is impossible to represent the exact quantity as a decimal; you can only give and approximate value. However, if you write a fraction that includes the symbol for pi, you are giving an exact quantity.
Pi is an irrational number. As such, it has an infinite number of digits.
Archimedes
I don't know how to represent the symbol here on WikiAnswers, but you can see it here: http://en.wikipedia.org/wiki/Pi
"Pi" is the symbol almost always used to represent the ratioof the circumference of any circle to its diameter.
the number is that you use for pi is 3.14 the number is that you use for pi is 3.14
The first person to use the pi (π) symbol was Welsh mathematician William Jones in 1706. He adopted it to represent the ratio of a circle's circumference to its diameter, deriving it from the Greek word "periphery" (περίμετρος). The symbol was later popularized by the mathematician Leonhard Euler in the 18th century, solidifying its use in mathematical literature.
Leonhard Euler used the symbol pi in 1737.
He was responsible for instituting the use of the symbol pi.
The symbol for pi (π), is the Greek letter that is the equivalent to the Roman p. It is the Greek letter pi. It is the first letter of the Greek word περίμετρος (perimetros) from which our word perimeter comes.