L. E. Gant
That they are evenly distributed, so the odds on a number appearing is the same as for any other number. This makes them unpredictable.
If you are asking about how they get generated, and if generated random numbers are truly random, they aren't really all that random -- they are in a kind of Gaussian (bell curve) distribution.
:-) once did a study of random numbers using accounting reports, and found that "1" as a digit occurs more often than any other digit....