How does custody battle over kids work?

If both parents (now divorced or separated) were great parents and normal shouldn't it be kids stay 2 weeks with one parent then 2 weeks the other? Rotating.

I always see cases "Am fighting for longer days of custody" or "Am fighting for kids." Is the system really unfair this way or is there real abuse going on in the home with one parent?

Also doesn't shift pattern make sense like "4 days with mum then 4 days with dad?" If both parents are within same town of course.

Am really curious and wondering.