Because it's good to know how well the project is progressing? It's good to know if you're on target? It's good if something happens to someone or multiple people that you all know what has and hasn't been done and is left to do?
That's the project manager's job. I shouldn't have to care if he's doing his job. It's a matter of trust and competence.
If I have a good auto-mechanic that I trust, I know I can drop off my car, he'll diagnose and repair the problem, do good quality work, and not rip me off. That enables me to have the peace-of-mind to not have to care about his work or how he gets it done.
I should have that same level of trust in everybody I work with so I don't have to care. I think part of the problem here is that the phrase "I don't care" is seen negatively in common use. I don't mean it that way. If I don't care about your work, it's because I know you'll do a good job (just like with my auto-mechanic). That frees my time up to work on what I need to work on.
If I have to care about your work, it implies I need to baby-sit you because I don't trust your quality of work or that you'll get it done on time.