You are held accountable for performance of your team, directly or not. I have worked with people who were a net negative - try to push their work on me, or just being an asshole in general. The productivity would have been higher if they were not there, and I would be much happier.
I guess to some extent it's about some aspect notion of "fairness". Another concern would be if their poor performance spills over into my own role and that prevents me from pursuing more interesting tasks that are more aligned with my growth/interests.
More practically, if I'm working at a place where my team has fixed head count I'd rather us have the ability to replace someone who's not contributing to the success of the team.
This can definitely be a burden, though on the flip side of the coin if the team is constantly churning due to cooking under a performance magnifying glass all the time, you still end up with lots of newbies who only just get up to speed enough to contribute for a bit before moving on because it's miserable. So sometimes it's better to deal with the problem you know, and just move them out of the critical path.
Because I actually want to make an impact at my job and constantly carrying idiots on my back is a horrible experience.
Thankfully my workplace is not unionized and I have great team members. Whether union or non-union workplaces can encourage that, I don't know, but that is an example of why an employee would care.
> Why do you care about this as an employee? You get nothing when somebody else makes the company more money.
Compensation often includes stock; so you do get something when somebody else makes the company money. And if nothing else, the company doing well may help you feel job security and be better able to make plans for your future.
You also may enjoy making the best product you can. When another employee gets in the way of that, it can make it harder to enjoy your work.