That’s my question: why?
It has to do with the force necessary to accelerate something to that speed. Acceleration is measured in meters per second squared, if you want to go metric. That same amount of energy must be represented in the velocity of said object that was accelerated to that speed, if we want to maintain conservation of energy. Why is it squared and not cubed? Because it’s not. Sorry, that’s the best I have.
“That’s my question: why?”
Only God can answer why.
But I don’t think that is the answer you are seeking.