Why doesn’t the iPad mini offer face recognition?

No one here would know.

Typically, Apple does NOT regularly upgrade the iPad Mini nor the iPod Touch.

You'd have to contact Apple wnd ask them.

Nobody can give you a proper answer about that, since we, just users like you, are not aware of Apple decisions/future plans.

Perhaps (hopefully) you might have that feature in next iPad Mini model.

Regards

Giulio