There is a strict process for deciding on the next host country of the Olympics. The committee responsible for this selection is known as the International Olympic Commission. First, the city completes an application to be selected for the next Olympic city. When the deadline has passed for the cities to complete an application for selection, the IOC looks at several factors of the city including is it can handle the Olympics and it can afford it. Many people think the athletes who come from the host country of the Olympics may have an advantage over other athletes based on the fact that they did not have to travel far.
Therefore, the athletes are competing during the same time as they were practicing and they are used to the time when competing.