How can a fertile soil lose fertility?

In several ways.

It can lose fertility when a crop is grown on it, and removed from the field without replenishing the soil with organic and/or chemical sources of nutrients.

It can lose fertility when it erodes. The topsoil is the most fertile part of the soil.

It can lose fertility over long periods of time, just by rainwater and snowmelt waters running through it. This slowly leaches out any soluble or partially soluble substances.