"I haven't either. Is there a good reason why we don't have one though?"
Because, no offense, I can negotiate my own pay and benefits better than you all can collectively for me.
For some professions, like teachers, police, etc, where everyone is doing the same exact job it might make sense to go union..... but you and me, although we are "IT", have two very different jobs and responsibilities. "On call" for you might be drastically different than for me. For better or worse a union is negotiating things very broadly and you can't just package "IT" with a bow and serve up the same one to everyone.
As an alternative, learn to negotiate, learn what questions to ask and what to ask for in return. Know what you're willing to accept, and don't be afraid to reject an offer. Ask for what you'll still be happy with 2 years from now. Try and avoid technically inept companies, and make sure expectations are set (i.e. this pager is for catastrophic network failure, not your home email account is broken).