[Sternenlied]

The answer is a psychological one.

So here - in a very basic form - it is:

Serial offenders (killers as well as rapists) usually don't team up.
They're usually not frightened by the thought of someone in the team leaking information or just being incapable of keeping their mouth shut. It's not the way their minds work. The killing or rape is triggered by a certain mental/emotional impulse and/or urge that leads those people to committing their crimes. They are usually driven by a mental image they want to accomplish or destroy and they want it to be their own. So there's no need to team up for more efficiency. It would actually destroy whatever they have imprinted in their minds so the thought of a partner usually wouldn't come to their minds, it might eventually even offend them.
Gang rapes however are triggered usually by something completely different.
They might be considered necessary means to strike fear into the heart of enemy gangs, to punish failure of a member or offending the gang. They might be an initiation ritual for someone to prove his "manhood" to the gang. Also they might be triggered by simple group-dynamic. Something I guess many of us might have encountered eventually. While the gang itself already offers improved self-conscience to its members I don't think a gang-related gang-rape should be considered a planned team-rape as it is the topic of this thread because the combined power of various gang-members together is usually aimed at increasing the members' confidence and/or the brutality of the crime and thus the delivered "message".
While the crimes of a serial rapist aren't usually about the sex itself, there are as well one-time offenders (remember the "I-was-drunk-and-she-turned-me-on"-guys) who go after the sexual aspect. But since they in general don't plan on raping someone, they don't team up as well. Drunken group dynamic (for example the mentioned football-team) is the most likely form of a team rape. There's usually no plan there either so it's not about increased efficiency as well.