They don’t.
Have you ever seen fog? Fog is just a stratus cloud forming at ground level. And overcast sky is just a stratus cloud forming very high above the ground.
Cumulus clouds have a flat bottom because the conditions necessary to form the cloud stop existing below a certain height.
Cumulobimbus clouds have a flat bottom and top for the same reason
Cirrus clouds have to form very high up in the air because they are made of ice crystals they need very cold air to form.
If you’ve ever been on a plane on a cloudy day, you’ll notice clouds both above and below you.
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