Because, unlike the PS3/360, the PC has been chugging along in terms of power over the years. We're not seeing the next generation of consoles from Microsoft/Sony for a good while now, at the very least a year, so at this point the only reasonable thing to judge the WiiU's new hardware against is a modest PC built within the last two years. It's the only window for us to see where modern technology is and where it's heading.
If I came in here and said my bleeding edge rig I built this year outperforms the WiiU I'd of course be a troll, because that's incredibly obvious and I'm just bragging for e-peen at that point, but I'm not. I have a modest one, from two years ago, and it's still running circles around what Nintendo's calling their "next-generation" console.
FWIW, I'm fully expecting the PS4/Durango to output in 1080p. I know the very thought gives the vapors to some GAFers, but those are my standards at this day and age after my experiences with that resolution. If I pay hundreds for new hardware, I expect them to at least perform as well as what I bought relatively recently (though to me 2 years isn't exactly recent), not worse. And I'm certainly not expecting it to perform "just on par" with something from 2005.