Why men deny cheating when caught red handed.

My SO of 6 years (living together) was just busted with 3 women in his phone. He knows them from Facebook, and they are well within meeting range. The messages were blatantly sexual. HE "neglected" to tell them he was in a committed relationship, and attributed not being able to meet them to work hours. He denied it all even though I was standing there with the phone and showing him each line. Why can't men (this is NOT my first go around with this sort of thing) just admit when they are caught and not make utter fools of themselves back-peddling, side-stepping, and lying badly? Especially since we are 50 and 51 and way to old for these silly kiddy games.