I'm not an expert, and I will try not to repeat things over and over that I've said on other threads, but I think there are a couple of things that make PI jobs different. They have fewer resources, and so there are simply fewer of those jobs, period. Firms are hiring tons of people every summer, where it's unusual that more than a couple of jobs per year are going to open up at any given public interest org, and sometimes none do at all, no matter how much they like you. They're only going to hire when they have the money and the need.And they don't generally have the resources to spare a bunch of their attorneys to do on-campus recruiting, so you generally have to do the leg-work of finding them yourself and applying directly. Some PI orgs do recruit or come on campus, but less so. Related to that is the fact that they're supposedly more interested in what you can do for them immediately - they're hiring you because they need to get the work done, not because they need to impress a client with your grades or school name. They don't have the resources to train you for two years before you start doing work that matters to them. So they are competitive along a different axis, essentially - you can have all the traditional sorts of qualifications, like law review, or good grades, but if you've never done a clinic, or any pro bono work, worked at a useful summer job, demonstrated that you care about this stuff and have learned something about how to do it, etc. they're not going to be that impressed.
So... my view of it is that if you want the super prestigious fancy competitive PI jobs and such, they want you to have both the traditional stuff that employers are supposed to care about (grades, school name, law review, whatever) AND proof that you are dedicated to this work and know how to do something. If you want a more typical PI job, they care more about the latter stuff - have you demonstrated a commitment to this type of work, and what actual skills can you bring to the table? So they are competitive, but in a different way. I think that's why it's hard to make a straightforward comparison.
This is my understanding, anyway. Anyone can feel free to clarify/correct me if this is not your experience or impression.