Englishclub: "Why do some nouns have no plural?"

[Aikuzo]

Englishclub: Countable Nouns, Uncountable Nouns
Why is this important? Why do some nouns have no plural?
dog/dogs, rice, hair(s)
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I tried to look around but I only saw guidelines about uncountable should only considered plural. So on nouns that have no plural, does that refer to uncountable? Or on countable nouns that only have singlural forms? (Ups are there such a thing?xD)
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I quit studying English years ago and have just come back to study the basics.

[Milanya]