I remember them asking about the soccer player and how he got signed up for Triple xXx program and all he does is kick, and June wondered if all the agents were athletes. Well, if I remember correctly, part of the reason Xander Cage was chosen initially was because the government had been having trouble sending regular secret agents to infiltrate the terrorist gang because they were so easily sniffed out. Xander Cage was a well-known badass xtreme sports guy that probably could pull off not being some square government secret agent. In the first movie, one of the main bad guy's henchmen even recognizes xtreme sporter Cage and it further helps him get closer with the bad guys.
So basically, taking famous athletes and letting them infiltrate gang organizations is easier than taking some random agent and trying to get them into the gangs.
Side note: in the first movie they actually tested out two other xtreme sports guys besides Xander Cage for the mission. One got injured and the other took off, leaving Xander and injured guy behind. Xander helped the injured guy while fighting off drug dealers, and that's how he made the final cut.