Primary proteins are also called polypeptides because

Successive amino acids are linked together through peptide bonds to form the primary structure of proteins.
(1) Though polypeptides have linear chains, it doesn’t specifically explain why primary proteins are called
polypeptides.
(2) Polypeptide is polymer of amino acids.
(4) Primary proteins can assume many conformations, but it does not directly relate to why primary proteins are called polypeptides.