Question

why is it said that matter is made up of atoms and not made of molecules

Answer 1

Because as suggested by the Dalton Atomic Theory atom is the smallest unit of the matter and molecule is group of atoms. Molecules are group of atoms held together by chemical bonds and have no net Charges. So it is said that matter is made up of atom not molecule

Answer 2

It is said that matter is made up of atoms as according to Dalton's atomic theory atom is the smallest unit and molecules are also made up of 2 or more atoms. Since molecules are also made up of atom, so it is said that every matter is made up of atom.