Plus the fact that those who do get the positions stay for the rest of their lives....
I also thing the distinct lack of information for nearly all captive wildlife species is one of the reasons why they require so many publications. Who knew one could be considered the leading expert of something just because they were the first to publish on it?
Totally true. However, the area is starting to move towards preferring boarded vets, especially large zoos that employ multiple vets and offer higher salaries. I'm waiting for the AZA to change the vet requirement from having a contracted vet to having a full-time veterinary staff, and I'm surprised it hasn't yet. It's difficult to manage zoo animals without daily veterinary presence, but then funding becomes an issue. A boarded vet will of course probably beat out almost any other applicant, but not all zoos can afford them. It's also becoming more popular for unboarded vets to apply for boarding later in their careers (you can either complete a residency, or have 6 years in a zoo for ACZM). I'd say you don't always have to be ACZM, though. There's ACT, ACVN, ACVP, avian, and reptile/amphibian that zoos also appreciate.
With the way things are going for zoos right now, I'd say we're at a fork in the road. Things will either go downhill, based on how society continues to perceive captive wildlife. If that doesn't happen, I see zoos expanding and hiring more and having their conservation efforts be much more appreciated. Hopefully then, veterinary teams will expand to allow us newcomers to get their careers started.
Also, it's generally frowned upon to spend time career-wise in an unaccredited zoo, whether you are a keeper or a veterinarian. The opinion on that varies from person to person, of course, but I'd say its the general consensus based on advice I've been given for both career paths. Unfortunately, AZA accreditation requirements automatically exclude a LOT of really great zoos that can't afford to meet some of the highest standards AZA holds. I think that stigma exists just because AZA-accreditation automatically gives the impression that the zoo is incapable of mistakes....but that's another discussion.