Distribution of executive powers between centre and states.

Why does the centre rely on state governments for implementation of most of the schemes and laws? Under what circumstances can centre give directions to the state governments? Has it ever occurred? In which situations states might impede the central executive power and what will be centre's response in the case?
Executive structure is same

If we have a parallel central government run administration, it would be cumbersome and inefficient

the central Indian Administrative Service is there for the purpose !!!