That’s because every natural number is the product of prime numbers (and 1). Indeed, that’s the definition of a prime number: a natural number is prime if and only if it is not the product of other natural numbers. Therefore all natural numbers that are not prime are the product of other numbers which must necessarily be either prime number or themselves the product of prime number (eventually).

Another way of thinking about it is the prime number are the prime elements of the set of natural numbers; i.e., those elements from which all other elements can be created through multiplication.