It's not because these new products are particularly bad, although Microsoft seems to have been... enthusiastic with the truth as far as the reality of Project Natal.
Nor is it because of the considerable effective price gap between a Wii and rival motion-control-enabled consoles. Presumably the idea is that instead of buying a 360 and a Wii, you'll buy a 360 with Kinect, and funnel commission exclusively into Microsoft's pockets. Whatever; I'm sure it can be made to sound sensible in those terms.
No, my problem with these systems is more basic: computer games were invented to entertain kids who didn't especially like jumping around getting sweaty in the first place. My first computer games were some of the first computer games, and experiencing them was like... well, it wasn't 'like' anything - which is precisely the point. The closest I can come are those dreams where you suddenly realise that if you concentrate in just the right way, you can fly: a surging, overwhelming sense of "Oh..... This is going to be cool."
Since then I have, to paraphrase my favourite monologue of all time, seen things some people wouldn't believe. I've docked in high orbit above the planet Lave, and avoided eating cake with psychopathic disc operating systems.
If I want to go bowling, I'll, you know, go bowling.
I've nothing against the Wii, you understand. But it's not a games console. Well, maybe that's a bit strong - certainly there are some genuine 'computer games' for it, mainly because Nintendo can't seem to help making them - but I don't really think of it as existing along the same axis as the BBC Micro, Amiga, PC, Playstation, XBox or DS. The Wii is primarily a device for pretending to do things that would be more fun if done for real.
The upshot is that Microsoft and Sony are jumping on a bandwagon that's being driven by people who don't like computer games. You can tell they don't like them because of how much the Wii has 'grown the market': if they liked computer games they would have already bought some. And when they're bored with virtual this and sim that, they'll just shove their pseudo-games-console in a cupboard and go do some gardening instead. They won't even care.
I can't help feeling that the millions Sony and Microsoft have thrown at development and the millions yet to be splurged on promotion would have been better spent teaching more people how much fun it can be to grab a mouse or a joystick or a gamepad and just sit the fuck down.