Why do athletes never help up people they knock over on the sidelines?

I've seen women, elderly people, even disabled people get knocked over on the sidelines of football and basketball games and 99 percent of the time, the athletes just walk away. I understand they are in a rush to get back out there, but it only takes a second to extend a hand and help up the unsuspecting person you just knocked over. I couldn't imagine under any circumstances not assisting someone I just knocked over back to their feet.