Why do transformers use so many turns?
When you apply voltage to the primary winding of a power transformer, some current will flow, even when the secondary is open circuit. The amount of this current is determined by the inductance of the primary coil. The primary must have a high enough inductance to keep that current reasonable. For 50 or 60 Hz power transformers, this inductance is pretty high, and you typically cannot get there with a small number of turns in the winding.