One reason would be to hide that there are no "Covered Patents". If what On2 always claimed about all the patents on VP8 belonging to them is true, then the only member on that list which would have any "Covered Patents" would be Google (and, by the way, is there a list of Google's VP8 patents anywhere?).
Another reason would be to hide that there *are* "Covered Patents". From what I have heard, On2 always claimed that all the VP8 patents belong to them; if one of the other members has a patent which might apply to VP8, that member might want to hide the fact so as to not "rock the boat".
And another reason, probably the real one, would be that a patent *might* apply to VP8, or it *might not*. Not having to identify the patents allows them to remain in that superposition of states indefinitely, while having to identify the patents would force them to collapse prematurely into whether it is a "Covered Patent" or not.