Why does it seem like clouds never lose their shape in the sky?

Depends on the type of clouds and type of day and where you are.

I was in a boat off the shore of Vernazza, Italy, on a bright warm day. Looking north towards a cape. I could see clouds forming in the sky, drifting south to where I was, and then the clouds evaporating as they moved south from me. I was amazed at it happening continually while I watched for half an hour.

Edit (referring to clouds from afar)

It depends on what type of clouds they are and how fast the wind is moving them around, but you can see all clouds move if you watch long enough, even stratus. In my experience the best for seeing significant motion are altocumulus at sunset.

You just aren’t watching them for long enough

My observation is quite the opposite. I see them morphing continuously, with the exception of those that are very high up in the atmosphere.