Because they are different, even if they are able to produce viable hybrids due to being closely related genetically, they would not interbreed. The reason is that hybrids would be somewhat intermediate between the two species and are therefore not as well adapted to the niche of either a chimp or humans. Therefore if there is a hybrid, it will likely not survive long enough to reproduce. That in turn means individuals that do not observe species boundaries when they mate will be punished by natural selection, by having their fitness (measured by the number of offsprings they produce and how many of those offsprings are able to reproduce). Therefore, an inappropriate, cross species mating may well produce children, but few if any grandchildren. Because of such negative outcome, many species exercise great care when looking for mates, because mistakes can be so costly.
That is why many species have special ways to identify themselves to members of their own species. If there is any doubt they are the same species, mating may stop. That is why hybrids between different species are so rare in nature even if many closely related species retain the ability to produce viable hybrids. Even though there is no one out there to make sure each species only mate with members of their own species, the invisible hand of natural selection makes sure they do just that.
The male and female members can interbreed. Some interbreeding is possible between related species but the offspring are often sterile, like mules.
