Because the noun that is made plural doesn't end with an s/ch/sh/z sound.
Adding an S, 'S, or S' to make these plural, possessive, or plural possessive does not add an extra syllable.
Bird/Birds/Bird's/Birds'
Year/Years/Year's/Years'
Dog/Dogs/Dog's/Dogs'
Adding the S, 'S, ES or ES' to make these plural, possessive, or plural possessive DOES add the extra syllable because it ends in one of those sibilant (if that's the right word) sounds.
Church/Churches/Church's/Churches'
Bush/Bush's/Bushes/Bushes'
Else/Else's (no plural forms)
etc