Why transformer is not fabricated on IC s?

Transformers can be fabricated in ICs...! You know how to fabricate spiral kind of inductors in ICs? We can thought of transformers as an extension to this scenario. In spiral inductor fabrication, first we are depositing metal strips in bottom layer, followed by growing the core above that. Then we deposit metal strips above this core in an inclined fashion and connect them using via so that the top end of upper 1st metal strip comes above the top end of lower 1st metal strip and bottom end of the upper 1st metal strip coincides with the bottom end of the 2nd lower strip. In such a way, if we grow the secondary also and connect the cores, we can realize a transformer in an IC...

Think of it....!