Since silicon steel has the advantages mentioned above, why not use the whole silicon steel as the iron core and process it into sheets?

This is because the flaky core reduces another type of iron called “Eddy current loss” . When a transformer works, there is an alternating current in the coil, and the magnetic flux it produces is of course alternating. This changing flux creates an induced current in the core. The induced current produced in the iron core circulates in a plane perpendicular to the direction of the magnetic flux, so it is called Eddy current. The Eddy current loss also heats the core. In order to reduce the eddy current loss, the iron core of the transformer is made up of silicon steel sheets which are insulated from each other, and the eddy current in the narrow and long loop passes through a smaller cross section to increase the resistance in the eddy current path, also helps to reduce the turbulence.