why do government gives reservation why this unfair thing is done by  the government

The Constitution provides reservations to citizens belonging to the Scheduled castes and tribes because these groups have historically been subjected to severe discrimination which has left them socially and economically backward. Hence, to redress them for the historical injustice they have faced, the Constitution offers them reservation. This is not unfair as they have suffered greatly at the hands of upper caste people and deserve the reservation that the Constitution provides.