The relevance here is many managers have experience with employees who seemed fine in interviews and barely met performance bars (or just flat out didn’t) despite working just one job.

It’s well within their legal rights (and a useful heuristic!) to not hire someone because they’re not comfortable rolling the dice on a candidate being able to meet performance criteria because they’re working multiple jobs. Because working multiple jobs is a lot harder than working one job on pretty much any metric one can think of, and is not a protected class or status.

They’ll also reap any blowback or rewards from doing so, including difficulty finding candidates, or not.

A manager considering it for something like a software dev position would just be hurting themselves, though likely only a little as I doubt ‘candidate wears plaid’ comes up often.

If it was someone hiring for a fashion designer position, or a public facing spokesperson position, plaid could be a huge plus or a huge minus (I’m guessing huge minus as of right now for most), and what the candidate wears and how the they present themselves relative to current fashions and norms is a huge and important element that the hiring manager would be incompetent to not consider.

That said, there are plenty of managers who are pretty incompetent.

You don't. But you never know that for sure. The whole interview process is just gathering data to make an estimate about whether or not the person will successfully perform in the role. Them having another job would be almost the strongest indicator I could imagine that they will not be successful.

As mentioned elsewhere in this thread, many people successfully work multiple jobs.