I think there are a couple of reasons why it is called 'dry' cleaning: 1. 'Dry' can be interpreted to mean no moisture (which is almost always used in reference to water), so it is sort of true. 2. It used to be actually dry. There are various ways that people have cleaned things through history without liquids at all, and I suppose in the clothing market it turned into a sub-market offering (initially) dry cleaning. Since then, we have discovered liquid chemicals that also work very well that can be used on dry-clean only clothes, but the market already had a name (and 'chemical cleaning' doesn't sound great.
Sleeping bags should be expensive and valuable (if possible) , you are trusting your unconscious body to them. I used cheap ones for ages until I tried a friends good quality one. When you realise the difference it makes and how much more you will use it, the price seems a lot more reasonable.