Here's why:
* Compound words are created by combining two or more words into a single word. Examples include "sunrise", "butterfly", and "high school".
* "White line" is two separate words, each with its own meaning. The phrase describes a line that is white in color.
While "white line" might sometimes be used as a specific phrase (like the white line in the middle of a road), it's not a compound word in the traditional sense.