My kids asked me this question and I couldn\'t really give a concise, understandable explanation.
So I\'m hoping someone on SO can.
Generating random numbers on a computer is like playing "Eenie meenie miney moe" when choosing who's It first in a game of tag. On the surface it does look random, but when you get into the details, it's completely deterministic. It's hard to make eenie meenie miney moe into a scheme that a person really can't predict the outcome of.
Also there's some difficulties with getting the distribution nice and even.