It is the ordering, sequence and juxtaposition of the individual shots which creates the vital impact of montage. A and B can be seemingly entirely un-related in subject matter. C is the implied meaning, which is never actually shown. Stresses the importance of form over content in film-making. It prioritises how things are shown rather than simply what takes place.
Film is art because it is goes beyond reality to become something more than reality.
It is an act of personal expression, just like a painting. The fact that it is unrealistic helps rather than hinders its status as a work of art. Realism , on the contrary, claims that films special strength is its special relationship with the real world. Film therefore should prioritise content and subject matter over form. The style of no style.