Dust caps are the valve caps them selves. That little bolt that goes up and down and screws in to keep air inside is not a dust cap. There are 2 little cutouts on each side of that bolt that still expose the inside to the dust. PICTURE . Those little cutouts are used when you pump the tire so that the air gets in when the pump pushes the bolt down.

Basically everything is hollow and exposed to the point where that rubber grommet closes the valve. If dust gets where rubber and valve meet to close the valve, there is a fair chance the dust will get in between rubber and the metal, especially when you pump the tire and push the air inside.

Hence the name valve/dust cap.