Because the fact that they can be the answer of a multiplication of a set of numbers other than one, means that they cannot be prime.
What is a prime number? A prime number is a number whose only factors are 1 and itself.
So for example, if you can use 2 multiplied 7 to get 14, then that means 2&7 are factors of 14. Since 14 has factors other than one and itself, then it isn't prime.