Question

Why cant we make transformers with 100% efficiency

Answer 1

because it breaks einstiens law . ie for energy to be produced there is loss in mass

Answer 2

 eeta=1-t2/t1
where t2 is temperature of sink 
t1 is temperature of soure.
working substance absorbs heat q1 and sends q2 to sink
hear t2 should be equal to t1 for the transformer to be 100 efficient
but this is impossible 
so we cant create 100% efficient transformer