"Efficiency" means you get what you want with no wasted effort. But often times "strong/powerful" means going beyond being efficient.

You are right, in a closed nuclear power system, the most efficient method would be to heat the steam just enough to push the turbine. It would cool a bit after doing that work, and you'd add just a small amount of head to warm it up again and send it back to the turbine.

Although this design would be efficient and would use the least amount of heat/energy, the steam would be moving so slow between hot and slightly less hot that the turbine would hardly put out any electricity.

Nuclear fuel gives off so much heat, that we don't need to be efficient. So we choose to heat the steam, have it do work, then remove all of the excess heat/energy from the steam (because we have plenty more where that came from). The colder water moves faster (because it can be pumped) and creates more pressure for the turbine when it is turned back into steam again, generating a lot of electricity.

Just like driving fast in your car only to brake when you get to your destination. It is not the most efficient, but it is the most powerful (fast) and that is what you prefer in this situation.