How do rock climbers not get tired?

Rock climbers are very efficient at pacing themselves, and only exerting the required amount of energy, and no more. They achieve this through training, experience, and properly reading the route.

Climbers do in fact get tired, just after a different amount of time than others, and they are able to handle it better than others.

In order for a climber to complete a route, which can sometimes very long and physically demanding, they must first understand the route and it's requirements, and what options they have that can help them finish it. Based on that, and on an understanding of what their own strengths are, they will decide on the beta that suits them best.

Knowing what they need to do on the route and properly reading it and coming up with the correct beta for themselves also allows them to find good resting spots. These resting spots are crucial for long and tough routes, as the climber can spend enough time in these resting spots to recover and to reduce the pump in their arms.

Climbers also train very hard in order to reduce the effects of the pump they feel in their arms, so that they are able to climb through it better, and to delay the pump from ever happening.