I imagine this comes from the fact that:
1. ObjC is only used on Apple's platforms (and the less popular GNUStep).
2. You'll have to use Xcode to write for these platforms (less popular alternatives don't add up here).
3. Xcode automatically inserts 7 lines of comments with // at the top of every file. Two files per class, 28 /'s per class.
If you code responsibly in ObjC there's no need to write comments. Therefore, if someone "has to" comment ObjC code there's something wrong with their practices.
The same holds for almost any language, but in ObjC that's just natural.