There are true random number generators in nature. They are given by quantum mechanics.
In computer science, you are of course correct. There are only pseudo random number generators. In nature, it’s however different. The most fundamental theory of nature we have, quantum mechanics, is intrinsically probabilistic. There is no way you can predict an outcome of an experiment, you can only measure the probability of such an outcome. And recently, it was even shown by these people that you can build a perfect random number generator when using this property of quantum mechanics!
And this extends to your criticism of evolution theory. Genetic modifications are driven by radioactive particles hitting tissue, which are emitted by nuclei according to the rules of quantum mechanics. You can for example never predict when a particular nucleus might emit ionising radiation, only with what probability it might do that in a certain amount of time. Hence, the driving force of evolution is fundamentally random, because quantum mechanics is.