Because running is an extreme sport. Sprinting for 2 laps straight is very tiring, takes lots if energy, and you have to use every muscle in your body and basically push until you can't feel anyhing. If you think its easy than obviously you suck at running.
shut the HELL up. Its not easy. As for what I can see all you've been doing is paying video games. Also running is one of the worlds hardest sports. Its way harder than it looks. Also running super fast for 2 laps straight around a track is super tiring and very hard on your body. Being a runner isn't "easy"