Why is the plural of deer the same as the singular form?

Was there a shortage of the letter ';s'; back in the day?Why is the plural of deer the same as the singular form?
The word comes from Old English so its origins are essentially Germanic, rather than the more recent French influences on the language. In Germanic languages, plurals are not always formed by simply adding an 's'.Why is the plural of deer the same as the singular form?
Oh deer(dear),They came one by one,all looking alike and went to back yard. when I went to backyard,they were so many but each one was deer.To date,they refuse but be only deer.There wasn't any need or any time to use 'S'.
Because Deer are so numerous, they never bothered to make a word to describe just one of them.

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