7 Answers

Erosion.

This is an interesting article.  http://www.seattlepi.com/national/article/The-lowdown-on-topsoil-It-s-disappearing-1262214.php

Rain.

Wind when it's dry.Rain when it's wet.

Erosion and people.

the slope of the land. The type of soil that it is. The underlying structure of roots and stones, etc.

A thieve in the night????

ROMOS

Nitrogen based fertilizers converts organic materials into a thin liquid that with lots of organic materials feeds plants very well producing enormous yields of whatever is there including weeds. After using nitrogen based fertilizers for a few years, along with herbicides to get rid of those pesky weeds and burning the dead weeds and other leftover roots and lawn clipping and leaves from the trees in the fall....then Nitrogen fertilizer again in time for spring flowers and then seedling flowers. But no matter how much water and nitrogen fertilizer you pour on your garden you may notice the garden soil packs down into very hard dirt clods with a rock like consistency between waterings. 

Repeating the same process year after year with diminishing results becomes so disappointing and your crop yields tells you something is wrong and you just give up on gardening convinced you have destroyed what you loved and looked forward to each spring. 

Saddened that you are not a gardener, convinced of your inadequacies you give up trying. That's the real loss...wanna-be gardeners who fail and stop trying. A little education and hands on experience is all that's really necessary to be a successful gardener. The chemical way is a waste.