Why do some people keep their pets, cats and dogs as their children?

Because people are social animals and emotional animals.

When a person is alone, if there is no specific thing to do and no emotional sustenance, a great sense of emptiness will crush people.

So you see someone with a hobby. When a person is forced or willing to live alone, she/he will always find something to pin her feelings on. Keeping a pet is one thing.

Pets are living things and can communicate with people simply, so they may feel like children emotionally.