decline/fall

Unless we change the nature of our education, we will see a decline/fall in formal skills in the working population

The answer here is "decline". Why can't I use "fall" here? It have the same meaning as decline right?

Loosely, yes, a decline of standards is a fall in standards.

However, formally physical objects fall, and abstract levels decline. Think of it this way: if a dam's sluice gates are opened, the water in the reservoir comes out through a waterfall, and the reservoir's water level declines.

PS. Even better: a stock market decline, a waterfall.