Before we look at why clouds are flat on the bottom, a little review on how clouds form, or exactly what a cloud is, might be helpful.
Clouds are condensed water vapor in the atmosphere, either in a liquid, or solid form.
Their bases are not flat, but may appear that way, because they are so high, or far away from us. The other kind of clouds, the ones composed from liquid water droplets, form in a variety of ways, but the end result is air containing water vapor is cooled to its saturation point, and then the water vapor condenses into visible water droplets, called clouds. When air rises, it cools, and at a particular level, it reaches its saturation point, and clouds form. That level constitutes the bottom, or base of the cloud. But the height of the cloud bases over a given area is fairly uniform, and the bases appear to be flat.