That doesn't work if you use a strict parser where superfluous fields are an error. It's quite rare, but there are good use cases for that kind of strictness.

Or when no fields are superfluous. For instance, when the code iterates over all the fields and does something with each, instead of just looking for known keys.

Or when the place you need to put the comment doesn't happen to be inside an object literal.

This is ugly.