I'm pretty thrilled that my sleeping bag needs a wash-- it means I've been using it well-- but I don't use dry cleaning often so a curious google led me here and my mind is blown. Are things not able to be wet by liquids other than water?! I don't get it.
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.