The number of games isn't even the problem either, most of the IPs you listed on the Wii are at least a decade old, whereas the ones in the Sony and Microsoft section are fresh IPs.
Of course they have fresher IPs. Sony and Microsoft are both much newer to console gaming then Nintendo, and Sony was able to rely heavily on third party titles. The original Xbox was pretty much just bought to play Halo and PC ports, since most of the original IPs MS started on it were either poorly recieved critically or sold poorly (anyone remember Azurik?).
The only things from the Xbox era even worth milking for Microsoft were Halo, Fable, Project Gotham, and Forza, and as we can see aside from PG they've all been milked pretty heavily this gen. We've already begun to see plenty of milking from both companies, and I garauntee you that now that Sony can no longer rely on third party exclusives, we will continue to see them milk their own successful franchises. We've already seen it with God of War, LittleBig Planet and Uncharted.