Well, I know that they are typically not. Professors all do different things, and some of them may be more upfront about the job market than others - but in my experience, and in the experiences of the vast majority of doctoral students and postdocs I talk to, their advisors (from college through the postdoc) were not honest with them about the market. Comments ranged from simply omitting important information to willfull self-delusion.
And I mean, they suffer from survivor’s bias, too. The ones teaching us - especially on the graduate and postdoctoral level - are largely the ones who made it, and the ones who typically had a smooth ride of it. My graduate advisor went straight from undergrad (also an HBCU, like me) to grad school to a prestigious 2-year Ivy League postdoc to an Ivy League professor position. In fact, his trajectory is one of the reasons I selected him as an advisor. My secondary advisor had a similar trajectory - undergrad, grad school, a postdoc - that he hated but that was nonetheless at a top university - a stint at an R2 in a desirable city followed by successively moving up to a prestigious R1 in an even more desirable city and then an even more prestigious R1, all of which were able to solve his two-body problem with his wife in the same field. None of the other professors in my department had struggled with employment - or at least, they didn’t discuss it.
And they rarely discussed job market stuff, either. Occasionally one of my departments would have a seminar on the nuts and bolts of it. The other department ignored it altogether, aside from the DGS sending around some job ads. But there was no talk of how hard the market was or what a competitive applicant looked like. The assumption was simply - you went to Columbia, you’ll find a job.
I think the prevailing attitude is that college students are adults and they’re supposed to do adult things. But although they are legally adults, they are brand new adults - they’re still adolescents in terms of developmental stage. It’s like expecting a baby who just learned to walk to run a 5K with you. That doesn’t mean I don’t think they should be responsible for the course of their own lives, but I’d argue turning to an experienced mentor is the responsible thing to do. You expect those experienced mentors to be straight with you about your career prospects.
Besides, when you’re 21 you have no idea. I was 21 when I graduated from college and headed to a PhD program, and I knew how to Google. I read all of the reports online about how the academic market was shrinking and it was difficult to find a job, and how the competition was increasing. I had decided that I didn’t want to chase academia at the time, for a variety of reasons. But it wasn’t because I thought I couldn’t do it. People who are 21 and headed to PhD programs are used to being the cream of the crop - even the cream of the cream. They’re the ones at the tops of their classes at Harvard or Amherst or Wisconsin or Ohio State; they have never significantly failed, by and large, and are used to being the best and the brightest. It’s easy to reason “Sure it might be hard for NORMAL people to get a job; it won’t be hard for ME to get a job, because I am special and uniquely competitive. I’m going to rock and roll in graduate school and be a superstar and roll into an academic job straight away.” In fact, I said if I was going to be an academic, I’d just skip the postdoc altogether. Ha!
You don’t realize concretely at 21 that when you go to graduate school you will be one of MANY who are also the best and brightest; you don’t know what that means in practice. And you don’t realize how small the ratio of worthwhile positions to applicants truly is. So yeah, I think 21-year-olds maybe have the responsibility, but professors have an even greater responsibility to inform their students, because they are mentoring from the position of experience and wisdom. If I saw a dude about to jump into shark-infested waters for a leisurely swim, I’d at least warn him about the sharks so he could decide for himself whether he wanted to take the risk.