Why is it that true random number generators are not in every computer?
A random number generator has to output random numbers that are uniformly distributed. The possibility  must be exact for all possible numbers. That constaintis very difficult to obtain with kind of noise generator circuit, however it is possible to make this with computation aka PRNG - pseudo random number generator. 

PRNG is used for all gaming machines, because it has to have a statistic log that prooves that hit numbers are uniformly distributed. Another bad thing with white noise generator is that is susceptible to the environment noise, it would be very easy to jam a slot machine with some external device.