[ajuc]

Because you can't get really random number from deterministic hardware (and computers are deterministic, more or less :) ). No matter what you do, running the same algorithm on the same input gives you the same data on regular computer. And the randomness (measured in Kolmogorov complexity [1] of the resulting string) is by definition smaller than the length of the generator program.

And truly random string is such string, that there's no possible program producing this string as output, that is shorter than the string.

On deterministic computers you can only produce pseudorandom numbers, with varying quality of randomness, depending on generator program you use, but it's sometimes not enough (esp. in cryptography).

[1] http://en.wikipedia.org/wiki/Kolmogorov_complexity