Why is it said that natural selection acts on phenotypes rather than on the genetic material of an organism?

It is said because it doesn't change the genetic material, but the outside of the animal when the species slowly adapts to it's surroundings and predators.

Example: A group of 10 birds are in a forest. One of the birds has a short, stubby beak, and another 3 have long, thin ones. The rest of them are normal. If the only food left after a natural disaster was flower nectar, then the 3 with the long, thin beaks would survive, and the other birds, poorly adapted for this outcome, would not live.

It doesn't actually change the genetic makeup of an organism but may slowly alter it's entire gene pool over time.