Two years ago, the possibility of careers having its own meta was raised. The question didn't get much interest, but the accepted reasons to not have a separate careers meta essentially boiled down to "careers is a little side-project, and meta.SO is a catchall for support."
Careers is definitely not small these days. There are currently 935 careers questions here on Meta.SO. For comparison, answers (!) has 1024, and meta.Programmers.SE has 1027 total. There are twice as many careers questions as retag-requests (461) and I'm sure everyone can appreciate how common those are.
Careers also strikes me as a very different beast than many other SO side-projects. Apart from its size, it's designed to be exclusive. There are apparently 52k users on careers, versus twice that many active users on SO. Why should over half the StackExchange community see meta discussion of something they're excluded from?
Apart from the exclusivity, Careers also doesn't feel directly related to the broad SE goal of building a good storehouse of Q&A knowledge. Careers helps match potential employees and employers, but that's a very immediate benefit. A question on SE today might help someone years from now.
TL;DR: Careers is a fine and worthy project, but it's distinct from StackExchange and should have distinct support.
I see from comments that there's some confusion over what I see as the motivation for separating Careers from MSO. I didn't explain that well.
I don't think that Careers is suffering by being included in Meta.SO. From everything I can tell, Careers questions are answered quickly and well.
I think Meta.SO is suffering by including Careers.
Meta.SO is the catchall for questions without a dedicated meta, but Careers is big enough to warrant a dedicated meta (or other support setup). Metas allow community members to discuss and determine the character of that community. Including Careers in Meta.SO hinders SO community members doing the community building that Meta.SO is meant to enable.
Jeff also mentioned plans for Meta.SE to come back, which will shift the problem from Meta.SO to Meta.SE and amplify it because Careers will be an even larger portion of Meta.SE.