Most of the brands using Android technology, Why they want use it?

Nowadays there are lots of mobile technology companies in this world but most of the brands using "GOOGLE's" products that means they are using "Android OS", they have their own company ,own brand ,own strategy, own designs, but why they didn't create their own OS and always using android?

As a person I think this way but as a company they didn't think about this, Why don't they think to create their own OS?

I need to know the reason about this, share your opinions...