It is to be said that iron losses are the same at all loads! so why we do that test by open circuit of the secondary winding of a transformer?

The answer lies in the equivalent circuit. If you don't open circuit the secondary side, then definitely some current will flow in secondary side which need not be rated value. When you see the primary side then some load component of supply current will flow thereby resulting in Ohmic losses. This fails the entire purpose of getting iron losses as a independent qty. Here the power drawn will account for Ohmic losses+ iron losses which is the general operating condition. hence to decouple the losses we conduct SC Test and OC test.