Question

why is hot milk not used for making curd

Answer 1

because in hot or cold milk the bacteria cannot reproduce quickly . while in Luke warm milk the bacteria reproduces quickly  this leads to formation of curd. but if the milk is kept for long time without refrigerating it makes the milk salty.

Answer 2

The biggest reason to heat milk to almost boiling before fermenting is that it improves the texture of theyogurt. During fermentation the bacteria consume lactose and produce lactic acid which causes the milkproteins to denature and coagulate trapping most of the fat