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My guess is they are exceptions in the English language.
You can say "5 fish" but not "5 fishes" and you can say "5 dishes" but not "5 dish".
You can say "5 children" but not "5 childs".
There are many exceptions just like the word "rendezvous"; it's pronounced "randevo".
Basically , language is taught including its rules as it is a means of communication.So it is taught just like that.There is no answer except that it is the language.
Plurals in English are generaly for countable nouns and they are either regular plurals i.e singular+s (which are more common), or irregular plurals: a limited list, some of them where the singular differs from the plural: child-children. Others where the word for both is the same as you mentionned in your question, though the words you mentionned. I'd suggest if not mistaken that since they are more likely to be found in cattles, herds or large groups, a unit would refer to the group.
Let's be honest, many English words, are not English words to begin with. Words which do not change in their plural form are basic English, carried down from Old English language as in the Anglo Saxon tongue, where words with S plurals, they derive from Greek, Latin even French.
