If certain reports and theories are to be believed... you're kinda already looking at the "Next Gen" of the PS3 and 360 with the Move and Kinect.
As has been pointed out, console development, especially if you're always trying to use the newest, most expensive technology at the time, has gotten MUCH more expensive in recent years. Right now, both Sony and Microsoft are trying to find ways to extend the lives of their current consoles to avoid the costly development cycle as long as possible (especially since both their units are still being sold for a loss).
This is similar to the tactic that Sega employed with the Sega CD and 32X add-ons for the Genesis (though we all know how well THAT ended up working out...).
Nintendo is in a different situation, as, while their tech is less advanced, every unit sold is still at a profit (which they kind of have to do as, unlike their competitors, they don't have other business arms to mooch off of to keep them afloat if things get dicey). And while sales are certainly down from previous years, they're still at a level where there's no real need to bring out a new system yet.
As for "why handhelds" let us not forget that both the PSP and the DS are older than the current consoles and the amount you can extend their life cycle by has reached its limit.
After the complete disaster that was the PSPGo, Sony needs to do something big to revitalize their hand held efforts in the US (they lucked into Monster Hunter being a ridiculously huge smash in Japan but that can only sustain them so long) so it's time to cart out the newest, most advance hardware they can.
As for Nintendo, the DS is in the position that, while it still sells, its reached the point where its sales are dropping enough that it's time to go for the next thing (and the tech they want to use for the 3DS is proven enough that they can sell units for a profit).
TL;DR version: It's not yet time for consoles to need an upgrade, for handhelds, it is.
