The simple reason is that it’s a loanword.

All nouns that are borrowed into English either form their plural with the

standard plural ending –s (the vast majority), retain the plural ending of the

donor language (e.g. phenomena, algae), or remain unchanged in the plural.

It is also quite possible for the same noun to employ

more than one of the above types of plural formation.

The word moose has its origin in the Native American Algonquian language.

Adopted into English by British settlers of North America in the early 17th century,

it comes from the Eastern Abenaki word mos, which also appears in southern

New England Algonquian languages, such as the Narragansett word moòs.

 

But why, then, do we say geese instead of goose, or gooses in the first place?

After all, geese is an obvious exception to the standard plural in English.

The reason goes back a millennium and a half to the beginnings of

Old English and to a sound change known as mutation (or umlaut),

defined as ‘a change in the sound of a vowel.

 

Back to the question that we started off with, why didn’t this mutation happen to moose?

As mentioned above, moose entered English via the Algonquian language in the early 17th century, long after the Old English vowel changes had happened.

Though vowel mutation in English had hardly faded into the background,

the specific process that gave us feet and geese had already

occurred hundreds of years beforehand.