Cloud droplets require a condensation nuclei on which to form; growth then occurs by deposition of water molecules from vapour.
Growth is limited by local supersaturation of the vapour
Growth rate decreases as droplet size increases
Cloud droplets are typically 10 to 30 m in diameter. Growth/evaporation can occur within a few 10s of seconds.
Rain drops are typically 0.5 to 5 mm in diameter, growth from the vapour would take several hours – longer than the lifetime of typical convective clouds.