As a millennial, I think a lot of my peers are, well, clueless when it comes to finding and landing jobs. Even knowing which resources to use and how to use them for finding jobs isn't inherently simple. The same can be said for interview etiquette. If they aren't taught it, there's a strong chance they won't know what they're doing.
At my university, there were certain programs that had VERY strong career services departments. Some of these programs even required you take a short course on finding jobs, resume building, interview techniques, etc., and the career service departments were VERY strict about students following proper protocol/etiquette. If you messed up, there were pretty strong consequences. Even some of the turd-students could become polished enough to land jobs (perhaps unfortunately). The Business and Informatics schools were very good about helping students land jobs, but only if you put in the effort.
On the other hand, I have a friend that went through a similar program as me, but not at the same university (IU for me vs IUPUI for him). His career services department was not nearly as involved in helping students (enough so that those less motivated or aware would likely miss it), and he was, as such, pretty clueless about finding jobs. I had to help him with resume building and interview preparation/tips. He's a good, smart guy with strong skills, but landing a job wasn't as easy for him for a while. Even my GF had a lot of trouble finding a job...same university as me, but her program (Biology) did not have a strong career services department, if it even had its own department at all...can't remember.
So, I think part of it is just general cluelessness and not really being aware of that enough to seek help if it doesn't come knocking first. And then some people just suck.