Fet is named field effect transistor,why?
A electric "field" is produced whenever and wherever there is a voltage difference.  Actually the field is always everywhere, but becomes non-zero when voltage differences are non-zero.  Since non-zero voltages are fundamental to how every electrical device works, all such devices can be said to work based on or somehow related to the "field".

Field effect transistors (FETs) work on a pricipal that uses the field produced by the gate in a way that makes the channel conduct more or less.  In bipolar transistors, the current that can be carried between the collector and emitter is modulated by the base current.  Of course there are electric fields inside a bipolar transistor.  The FET was named such because the of how the static field, as apposed to current, plays a central role in the operation of the device.

It's a naming scheme, which is not meant to be a detailed description of how the device works.