I don't know whether it's mandated, but I think that a lot of gamblers would want it that way. They want to be able to influence the game. The outome will still be completely random if the other sources of randomness are good, but the outcome still depended on the gambler's timing.

There has to be something that advances the PRNG to the next output. What's the alternative, advancing a single step each time a random number is actually needed? That also has (potentially much easier to exploit) vulnerabilities.