Here's why:

* Polar vs. Nonpolar: Salt (sodium chloride) is an ionic compound, meaning it's made of charged particles. It's considered a polar substance. Vegetable oil is a nonpolar substance, meaning it doesn't have a positive or negative charge.

* "Like dissolves like": The general rule of thumb is that polar substances dissolve in other polar substances, and nonpolar substances dissolve in other nonpolar substances.

Since salt is polar and oil is nonpolar, they don't mix well. The salt particles will simply settle to the bottom of the oil.