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.

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.

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.