The advantage of interviewing employed people is that you have a better chance of hiring someone that is competent. That's also the reason, in a one local company's case at least, that your list of employers is looked at, as some companies hire a very high percentage of frauds.
One local employer, always looking for people, and not a bad place to work, has 3% of people pass the on-site interview, and it's not especially onerous: It shows you have general programming competency. Everything their exercise asks is something you'll use on the job. But most people that apply fail, because there is a local glut of people that know less than a proper entry level candidate, but ask for senior level salary. Those people tend to be the first to get laid off, and the ones that stay unemployed the longest.
There are great unemployed people out there sometimes: A SF startup that is 95% remote laid off 60% of their staff three weeks ago, and anyone would be fortunate to get that unemployed talent when they can. HR has to shift through a lot of unemployed people that you probably don't want to hire to find a great programmer, and a great programmer probably already has a network that gets them snapped up in days.
It's absolutely not fair to the good programmers that end up being unemployed for one reason or another, but given my experience interviewing, it's not hard to see why someone unemployed and without a good network gets passed up far more often than they deserve.