They exist in many places but are a result of the existing social networks within the colleges. Some colleges have IMs set up around dorms or the greek system. Some are open and people form teams and register. Some, like Yale and Harvard have deep IM traditions and are extremely well funded and organized and are part of a year long competition between the residential colleges/houses akin to the rivalries at Hogwarts. Where schoolwide social tribes already exist (like Yale and Harvard), IMs foster great unity and pride and competition.
The fact is most other schools are “join if you can get a team” – which naturally only appeals to the most devoted cadre of sports enthusiasts. At Yale and Harvard, you may not play – but your college reputation is on the line so there’s fervent enthusiasm to win and compete well in IMs.
At other schools, the IM sports champion team gets some t-shirts and bragging rights with their boyfriends and girlfriends. IMs simply aren’t set up to be a unifying factor at many colleges. It’s a valid idea to explore however. It’s not cheap and colleges are tightening belts, however. Tell them to visit New Haven and Cambridge and get a sense of how vital they can be to the college spirit.