The reason is that any random number generator on a computer will produce what are called pseudo random numbers because it will always produce the same sequence of numbers even if you write a program to generate the first 100 billion digits of pi or some other irrational number you need to start it from a different place each time if you are not to generate the same sequence of numbers.
In fact incorrectly implemented RNGs is one of the reasons that WEP for 802.11 is so weak.Well not badly implemented but badly understood.