Transformer design has to optimize a lot of different variables. And I am not an expert. But the transformer primary needs to have enough turns so that it acts like a big inductor and prevents excessive current from flowing. If you take a particular transformer core, you can either wind the primary with many turns of fine wire (high voltage primary) or wind it with fewer turns of thicker wire (lower voltage primary).
But in both cases you MUST fill the whole winding area with copper to make sure the transformer will work well and have low resistance and handle its rated power.
You cannot just put one turn of thin wire. That is basically a short circuit, not a transformer. So, if you choose a transformer core based on power requirements, then choose your operating voltage, then you can calculate how many turns are needed to prevent oversaturation of the core. THEN you choose a wire diameter which will fill the available space efficiently.
That is more or less how the design process works. Once the primary is settled, you choose the number of secondary turns to get the desired secondary voltage. Once you know the number of secondary turns, you choose a wire diameter that fills the space effectively.
Generally, you end up with a lot of turns on your primary for 50 or 60 Hz power transformers.