Here's why:
* Salt is a granular substance: It's not a group of individual creatures or things that would naturally form a collective.
* Quantity is usually expressed directly: We use terms like "a pinch of salt," "a bag of salt," "a spoonful of salt," etc., to indicate the amount.
If you need a creative term, you could use "a sprinkle of salt" or "a scattering of salt," but these aren't standard collective nouns.