Why are leads of primary and secondary plugs in step down transformer shorted?
This is either an autotransformer, or a regular transformer wired as an autotransformer, not an isolation transformer

The manufacturers of this transformer-unit most likely used an autotransformer, as all they are after is a voltage step-down, not isolation.  Even if they did use a two-winding transformer, they have wired it with a common neutral between the primary and secondary sides, which defeats any isolation provided by the transformer itself.  This is unlike an isolation transformer, which has no galvanic connection between current-carrying conductors on the primary and secondary sides.

The reason this is done is because it's less expensive (less copper) to make an autotransformer, and also less expensive (less insulation and testing) to make a step-down transformer than an isolation transformer.