Due to the high performance and strength of GROUT, why we can not use it for casting complete structural member (beam,column,..etc) ?

Mainly due to COST. This is not cost effective for a given project. However if you are aware for heavy machine foundations grouting below machine skids are done by grout.

It is also to do with its inherent properties of non-shrink or expansive cement types being used in the manufacturing of cements.

cost and high surface tension