Food was often sold by the dozen, and served by the dozen. Tables sat four, and baking pans that baked six or twelve were convenient. And it was a multiple of two. Bakers and grocers could price things by the dozen, and sell more, just like bulk pricing gets people to buy more now. Whether you were serving two, three, four, six, or eight, you could use a dozen (two dozen if it was eight).
The more common a word is, the more likely an irregular form is to stick around. So the verb “to be” remains irregular, and the plural of common words like “child/children” retains an old form.