Why do I keep hearing these people who claim to know nothing in interviews got the jobs?

I am one of those people. Always have been in any type of interview/audition situation. I think part of it stems from not wanting to ruin anyone else's hopes of getting the job, and another part not setting your expectations too high so you won't be disappointed. I have nothing to gain from telling anyone I killed it in an interview. If I tell you I messed it up, my expectations for myself are less, and you might feel a little more hopeful about your own self. And I don't come across as a brag or a jerk.

If you're crushing the questions/assessments but not advancing, maybe its a fit thing. Do you come across as someone they wouldn't want to be in the office with for 16 hours/day?

you're probably vastly overestimating the extent to which you're "nailing" the questions, while underestimating how you are coming across in the "soft skills" department

For most places, technicals are more of a check in the box. In the end, it really comes down to fit and a little bit of luck. Fit is hard to describe, but many times as soon as you walk out of the room, you can tell whether you hit it off really well with someone or not.

You might not be coming across well? How does the interviewer respond to your answers, is there much positive affirmation?

I had an interview at one of the top PE houses, a few days back, and the interviewer just didn't seem at all interested. I ended up asking most of the questions otherwise we'd have been done after 10-15 minutes. Either she really didn't want to be there or made her mind up about me pretty quickly. Sometimes you just don't click with people.

I have to agree with most of what's being said above - technical skills in an entry level position can be learned, your resume will give them an idea of your ability to learn. Try to focus on being more personable and less robotic - they're much more likely to want to work with you if they have a genuine interest in you and not just your work.