They may want to quit, but feel they can't because their spouse, parent or someone else wouldn't approve. They may want to do something else, but not have the energy and mental focus after working all day to successfully start something else.
I spent years trying to figure out how to make money in an entrepreneurial fashion. As long as it was a side gig while I was a homemaker or corporate employee, I couldn't figure it out. It was a different mental model from what was paying my bills and I couldn't figure out what on Earth I was doing wrong.
I figured it out once I quit and needed the money to literally eat. I still don't make tons, but it's no longer just completely mystifying and unsolvable.
Jeph Jacques, author of Questionable Content, was fired and had a part-time web comic. He began selling tshirts via his web comic to help make ends meet while job hunting. A year later, he was being supported full-time by his comic and had stopped job hunting.
Years ago, one article claimed that he was the highest paid web comic artist in the world.
It's good to be sensitive to the fact that being fired can be devastating. But it isn't always.
You need to find objective performance metrics for when it makes sense to let them go. You can't give them endless second chances.
That doesn't actually work for either party. It convinces bad actors that their current performance and behavior is acceptable.
That actively makes their future worse by denying them information concerning what it takes to meet the standard. Not firing them because you feel sorry for them implicitly says "Your performance is sufficient."
If the truth is that they are only keeping the job because you feel sorry for them, you aren't doing them any favors. You are lying to them about what adequate performance involves.
Coddling people actively cripples their professional and personal development. If they are "a charity case" that you are helping because you feel sorry for them, it's better to literally give them charity rather than give them the message that their shitty performance constitutes adequate job performance.
That just creates monsters with an excess sense of entitlement who believe they are better than they really are. It doesn't foster excellence.
It's hard enough to see yourself clearly without being de facto lied to in that manner.
It's also a huge disservice to those people who are actually earning their way. It will not be lost on them that they get less for more virtue. This is a recipe for mutiny.