It is due to the law of large numbers, and the statistics of many particles.
Work Step by Step
Atoms and photons are subject to the laws of probability and one cannot predict how any one of them will behave. However, for large groups of them, we can be much more certain about the results. For example, in a double-slit interference experiment, we have no idea where the next photon will hit the screen. However, after trillions of them have passed through the slit, we can correctly and very accurately predict the intensities on the screen. To cite another example, if we toss a fair coin, we cannot predict whether that one coin lands with heads or tails showing. However, if we toss it a million times, we can be more than 99% certain that the number of heads will be between 498,500 and 501,500.