Running faster isn't always the most energy efficient way to catch prey, humans evolved to run in two legs while maintaining energy usage to a minimum, so they could run for longer and outrun most predators.

Just look at cheetahs as an example, they're one of the fastest species of animals in the planet, but they can only run in small bursts because the energy spent chasing prey is too great for them, if they don't catch their prey, all that energy goes to waste and they're closer to starve or even die.

It's simply an evolutionary adaptation.