Hi why our current video games have bugs & glitches?

Hi what is the history of bugs & glitches? Do game developers are them or that's how is develop? Right now I'm playing Fallout New Vegas & I just throw a hand grenade to the 2 Legions who are trying to kill me & when it exploded, they went about 1,000 feet in the air. What cause games to glitch & have bugs. Thank you.