It's just one of those funny irregularities in English
Many words, such as deer, moose, and sheep, are spelled and pronounced the same way in both their singular and plural forms.
Other interesting words with this property are congeries, kudos, premises, shrimp, bison, shambles, series, and species. Fish can be both singular and plural, yet fishes is also a correct pluralization of the word.
Other irregular plurals originated in the highly inflected Old English:
goose - geese
mouse - mice
ox - oxen
tooth - teeth