Why do telephoto lenses have a higher focal length?

Question

Why can you not just use a lens element with a higher magnification instead of projecting the same image with the sensor further away from the lens?

In: 2

Answer 1

With just higher magnification you get more distortion. Telephoto lenses use a series of elements in groups to enlarge the scene with as little distortion. While the mm is the distance between the back of the lens and the sensor, there is a lot more to it. https://www.digital-photo-secrets.com/tip/215/what-does-the-mm-mean-on-a-lens/