Attending sporting events is often a treat for the fans. They are away from work and act crazy, often painting their faces, drinking too much, and doing other things they would not ordinarily do. Prostitutes know that some fans will be looking to keep their good time going and do other things they wouldn't typically do.
It's the perfect customer base! Get a few thousand drunk men together and all cheering because of the sports, and you've got a great chance to make yourself some money before the game is over. I'm sure there are also prostitutes who attend simply to watch the games as well, no reason they can't get into it.
Prostitutes are often at sporting events. Wealthy players and other people associated with the sport often hire them. They are typically high end escorts, but that does not change how many of them there are or that it is a great place to pick up clients and make some money.
There is no reason not to believe that there are prostitutes who frequent sporting events. Sex workers are people just like any other, and some of them may enjoy sports just like others who do. As such, they would be just as likely to be at these sporting events as anyone else; their professions do not enter into it.