Why is that? I assume you mean a server so far away that the number of hops (switches, etc) is higher on land than over the air? Without a large hop count difference, I don't see why air vs ground fiber would be terribly different. Air takes a direct path but with LEO satellite there's some height, and fiber takes a slightly less direct path but without height and with switches.

Light only moves at about 60% the speed of light in glass fiber. (Specifically, it goes at C / refractive index)

Why is the satellite --> server end lower latency than standard fiber?

The speed of light in a fiber is only about 2/3 of the speed of light in a vacuum. Plus satellite to satellite is line of sight vs zig-zagging around property rights-of-way in the ground.