Of course they don't exist to help you plan or execute a career. Most of the organizations with booths are bottom fishing, looking for enough willing and able employees to fill established gaps in their companies. This is hiring on the hoof, wholesale filling of average jobs with people trying to be average. Planning a career at a career fair is a little like looking for a soulmate at a singles' bar.
And fair? Hardly. Because there are no average people, right? There are average jobs, certainly, average in that they require people to fit in, do what they're told and follow the manual. I'll grant you that those jobs need to get done, but I'm not sure they have to get done by you.
By the time a job opening hits the career fair, it's a job you don't want. And by the time a job seeker is walking down the aisles, standardized resume in hand, it might be too late for her to find a job that's worthy of her.
Here's to a new, better sort of career fair, one that's selective, interactive, long-term and both career and fair.