Making casual games isn't easy, even for AAA teams. It's a completely different standpoint / mindset, and all the expertise and skill of said AAA team developers are likely focused on 'next-gen' and 'core' game development. Casual development requires a whole different way of thinking and developing.
And that is just about "casual" game development. Just think how incredibly hard it must be to make a game that EVERYONE, every target group, in every region in the world can play and have a fun time doing so. A game with truly universal appeal. Nintendo is making such games for everyone, not just casuals.
Also keep in mind that first party software
sells the system it's on, not the other way around (so the Wii games (from Nintendo) made the Wii succesfull, not that the Wii made Nintendo's Wii games succesfull). The hardware is just a way of playing the game. The software is made for the system, but in Wii's case the system is made for creating unique games. Neither Microsoft or Sony can replicate this; they lack the hardware- en software development teams / experience / mindset to do so. Their hardware is made for 3rd party developers and the games they want to produce. Nintendo makes hardware for Nintendo and Nintendo games. Nintendo is by far the biggest and most succesfull gamedeveloper for a reason, you can see it every generation. Even at their worst they still sell insane amounts of software. So imo, it is very unlikely that someone can simply reproduce anything about the DS and Wii and be wildly succesfull by doing so. Hell, not even Rare could do a good job (imo) of copying Nintendo's Mii's.
So far this generation, only one publisher has had 1 game that truly resembles Nintendo's efforts with these universal / casual games. Ironically, it's Sega with Mario & Sonic at the Olympic Games; a game that is co-developed with Nintendo. Although it has to be said that Guitar Hero comes close