why we can't use term aas

AAS is also a congruency criterion. That is, two triangles are congruent if two pairs of corresponding angles and a pair of opposite sides are equal in both triangles.

 

This is so as when two angles are equal in the triangles, then automatically the third angle will also be equal (since, sum of all angles in a triangle is 180 degrees). So, then you can think of ASA congruency rule also.

 

In all, there are 5 congruency rules:

SAS

ASA

SSS

RHL

AAS