Here's why:
* Alliteration is the repetition of consonant sounds at the beginning of words that are close together (like "Susie saw seashells").
* Onomatopoeia is the use of words that imitate the sounds they represent (like "buzz" or "crash").
The sentence "Susie saw seashells at the seashore" doesn't use words that sound like the actual sound of seashells.