Why can we not say brackets instead of parentheses ?
Brackets and Parentheses are not one and the same thing. Unless qualified, the term brackets may refer to different types of brackets, such as the square brackets [ ], the round brackets ( ), etc. However, in common usage, the term brackets is used to refer to the square brackets. These are used to enclose explanatory or missing information within a quoted text.
Parentheses, on the other hand, refer to the round brackets ( ) only. They are used for indicating additional information. The enclosed material can be omitted without affecting the meaning of a sentence.