They're kind of like games that are based off of licensed movie IPs, they're made to appeal to a particular demographic, in this case fans of professional sports and they are essentially sports simulations. Each year, there are several released based on the very same sport. Like 3 football games, 5 basketball games, 4 baseball games, etc, and they're all very much alike in their game-play. They were made for fans of a particular league of sporting events. Most gamers don't really buy them, so they start to pile up and become really worthless over the years.
I actually do like sports games, but I like arcade style or casual sports games that are more consistent with the principles of action based game-play than actual sports. For example, NBA Jam, 1080 Snowboarding, Wii Sports, or Punch-Out!
Collectors get tired of seeing these licensed sporting games pile up in place of the kind of games they actually want to find. Everyone knows the frustration of hoping for that next gem, and coming across a picture of John Madden's stupid face.