It’s the way they’re made. Windows are manufactured with desiccant (i.e. drying beads) built into the spacer. The constant heating and cooling of the airspace causes moisture to accumulate in the desiccant. Once the desiccant is saturated with moisture, and the outside temperature warms up (daytime), the moisture evaporates into the air space. When the outside temperature reaches the dew point (evening), fog and condensation will show up. At really cold temperatures the condensation will show up as starry patches (frost) on the inside air space of the window.