Fame and/or to sell their stories and importance to people that don't know any better. By the time Henry Hill died you'd have thought he was Acting Captain of that crew. The last book Hill wrote he claimed John Gotti himself showed up in a leather trenchcoat and shot Tommy D to death in a restaurant with a silenced .357 magnum. He knows it's not true, he even invented a new gun, but why not include the most infamous big name mobster in your story?