Why is glycine different from most other amino acids in a peptide chain?

Glycine is unique among the amino acids found in proteins because its $\alpha$-carbon atom is not asymmetric.1

In all other amino acids, the $\alpha$-carbon is attached to four different chemical groups, which allows them to exist as two distinct spatial isomers that are mirror images of each other. Because glycine lacks this asymmetry—having two hydrogen atoms attached to its $\alpha$-carbon—it does not exhibit optical activity. Consequently, while other amino acids can exist in different geometric arrangements, glycine is the simplest amino acid and remains optically inactive.12