In my experience, there are a couple of factors involved in this:
Management doesn't really understand what unit testing really is, or why it has real intrinsic value to them.
Management tends to be more concerned with rapid product delivery, and (incorrectly) sees unit testing as counterproductive to that goal.
There's a misperception that testing belongs solely in the pervue of QA. Developers are coders, and can't write tests.
There's a common misperception that management will have to spend money to do unit testing correctly, despite the fact that the tools are freely available. (There is, of course, the developer ramp up time to consider, but it's not really prohibitive.)
Will's answer will round this answer out: It's very hard to determine the value of test code (edit jcollum)
Naturally, there are other factors, but those are just what I've run into so far.
The first real software company that I worked at was all about the unit testing (NUnit). I don't know that we were real sticklers for it back then -- I have no idea what our code coverage was like and I was writing most of the unit tests. Since then I've run into some companies that do lots of testing, but it's chair testing: relies on a person being there, has low repeatibility and low chance of catching bugs. The other attitude is: it was something they wanted to get going with "in the future"; basically when money falls from the sky.
I miss unit testing -- it just makes life easier. But I'm finding that when I look for a new job, unit testing is either something that companies would like to "get going with" in the future or something they don't do at all (uhh, it's been around for a while now!). I'd say that 60-75% of the job reqs I've looked at over the past 2 years have not listed unit testing at all. I can only think of one or two that had unit testing experience as a requirement (for a mid-level developer position).
So the question is, what's missing? I think it makes people more productive, but that's only after spending a solid amount of time actually doing it. Aren't there any good studies about the cost savings of unit testing? Is it the type of company I'm looking at?
Edit: even though the title is a bit devils-advocate, I consider myself a unit testing proponent.