Because is more time efficient and because it stimulate your body to have stronger and sometimes bigger muscles. Even sprinters from before weight lifting was use as part of their training had more muscle than long distance runners.
When you train harder or go faster it increases the intensity of the stimulus in your muscles to be stronger, powerful, faster and bigger. As your body adapts to a certain speed like for example in running, your muscles get stronger and they become efficient at that specific speed. In order to stimulate your body to keep getting your muscles stronger, the stimulus needs to be higher.
Sprinting also increases your post exercise oxygen consumption which means more calories burn when you are resting and that’s when your body uses most of its energy from fat sources. This is for the Fat Burning Zone Lovers.
Why not go longer time?
If you keep the same intensity for a longer period of time it will only stimulate your body to use energy and find more energy sources, like muscle. According to research your body doesn't start using muscle as energy until two hours of aerobic training, but after one hour of continuous training your testosterone levels decrease and cortisol levels increase. Cortisol is a stress hormone that basically causes your body to use muscle tissue as energy and increase body-fat storage.
In the past scientists used to believe this happens after 4 hour of aerobic training, but after more research was done it turns out is only 1 to 2 hours.
I believe the time would be different for everyone depending on their level of training but I wouldn't like my testosterone levels decreasing when training or my cortisol levels rising.
I hope this answers the question
Rick_CSCS
