Hello there. Their is no such hard and fast rule that if you have done your mbbs you have to work in any government hospital. Many doctors do prefer government hospital since their the government factor and they are provided with all the government facilities but it totally depend on you how you want to utilise the knowledge which you have gained in your study period of time. So at the end of day your objective is to help other as being a doctor and you can do that the way you want either being a government hospital or open up your own clinic and serve the one which are in need.