A person is assigned work, and they need to be able to do it, without being a deadweight on their peers. At the very best, you're just dumping a SWE salary's worth of dollars into a ghost. I've never worked anywhere that didn't have plenty of work, and so that's an eng we could have had, but effectively did not.
IME these people talk a lot of talk, but they don't deliver contributions. Like the simplest script could take a quarter? I don't endorse code metrics, but, e.g., 1 PR / quarter (of really simple stuff), unless that PR is blowing my socks off or had some complex amount of research or some reason behind it, is underperforming.
You expect negative returns for a new employee like the first 6–12 months. This is well after that, where it's clear that they're not putting in the effort to understand the systems around them, or they lack fundamental skills (like experience writing actual code), or more often, both.