Because 1 is not a prime: "A prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself." https://www.wikiwand.com/en/Prime_number

Funner definition is "positive number with exactly two divisors". This reduces the amount of special casing

The reason:

"The uniqueness in this theorem requires excluding 1 as a prime because one can include arbitrarily many instances of 1 in any factorization"