When a working mother is not available, isn't it better the child's grandparents takes care of the child instead of an uncaring babysitter?

What's best is up to the parent(s) of the child. She may prefer the babysitter for valid reasons, no matter how much the grandparents would like to be chosen for this.

I used to babysit, and I was not uncaring.

a) Not everyone is lucky enough to have parents nearby willing and able to help with grandchildren, and b) The parents would likely not select an "uncaring" babysitter. They would mostly likely either hire a nanny or find a daycare center they liked that they felt would take good care of their child. 

Totally depends on the grandparents... there are plenty of babysitters and nannies who are far more caring than some grandparents. 

My parents do not approve of stangers taking care of the babies

Not necessarily. My parents caused a great deal of trauma in my childhood that I am still working through as an adult. I would not want my child to have to go through the same.

Grandparents already did their time raising kids and they EARNED the right to relax in their golden years not chase after screaming rug rats all day.

Your question is biased because you refer to babysitters as uncaring, unless you're referring to a particular one.  Yes, it's best to have a close family member babysit if the are willing and able to do the job.  And it doesn't hurt a child to be cared for by a capable and caring babysitter.

Not necessarily. Many elderly people have serious health issues that preclude them from being reliable caregivers for young children. Some have issues with mental illness, too. And what about those grandparents who have absolutely zero interest in taking care of their grandkids? There are plenty of people in that category too. Most babysitters aren't uncaring, either. It's wrong to make assumptions about people that you know nothing about.

Grandparents are not always in good enough health to watch after a young child.

Also - just because they raised a child doesn't always mean someone was a "good parent".  There are many people who would never let their parent watch their child because they don't want their parents to treat their child the way their parent treated them.

Personally - I did try to use relatives for watching my children instead of strangers.  Aunts and Uncles also can make good babysitters.