“Because it’s there,” respond climbers automatically, echoing the legendary George Mallory who may or may not have climbed to the top of Everest before dying in the attempt.
It’s an inner desire and purpose, which you can’t always express in words to someone who doesn’t have it. It’s like asking, “Why do you fall in love?”
There’s a great book called Into the Silence which tells of Mallory’s quest for Everest and the story of the World War 1 veterans that accompanied him. He hit on a great point that to truly understand the story you need to know a few things about the Great War.