Sheep is an uncountable noun, like fish, that's why.

"Sheep" is one of those English words that is both singular and plural... in other words, "sheeps" does not exist in English. Other examples are "moose," "aircraft," "deer" and "eyeglasses."

Sheep is an uncountable noun.Each language has its magic that makes it unique. If everything is regular , then no need to make an effort to learn a language.

first, the word "sheeps" doesn't exists. "sheep" doesn't change its spelling either used as singular or plural. the only way you can tell that "sheep" is just one or more than one, is by using determiners such as "a", "the", "herd", "two...three...four...", etc. the most important reason is Sonia's answer. :)

You can use sheep for singular and plural(it's an irregular plural)