It probably seemed like a good idea at the time—close to 10 years ago. Changing the default to false would be a breaking change.
Pluralization is performed by gobuffalo/flect, which is limited to English. This Go module is also used by these template functions:
inflect.Humanizeinflect.Singularizeinflect.Pluralize
If someone suggested adding this feature (and the associated template functions) today, I would vote no. Pluralization is complicated, based on language, and should be performed via Hugo’s i18n (T) template function.