Firstly, it's a video game set in a fictional universe. Secondly, you don't respawn in real life when you die. Thirdly, hand grenades don't magically reappear in your pocket once used. Fourthly, we don't have super powers. Fifthly, you can't carry a rocket launcher, sniper rifle and rifle and still be mobile. Sixthly, you can't sprint indefinitely. Finally (for now), see point one.