We could also ask "if the sky is blue, then why aren't the clouds blue also?".  Well the answer lies in the same exact concept that you've been learning about, "scattering" of the light.  So far we have learned that there really isn't anything "blue" in the sky besides the blue light waves.  Likewise, there also isn't anything "white" in clouds.

Clouds are made of on tons of transparent water droplets or ice crystals.  So why can't we see through them?

Well these droplets are huge compared to the wavelengths of light.  In comparison a cloud droplet is about 50 times bigger then a wavelength of light so they scatter the light pretty evenly.  If all the light waves are scattered evenly, the combination of all the colors appear white.  (Rainbows, Halos, and Glories. p.126)