Because you can't assemble your own smartphone whereas you can assemble your own PC. Brand-name pre-built PCs are idiotically expensive and come loaded with bloatware, a mandatory warranty fee, and so on. Also, my custom-built average-ish gaming PC cost me $570 roughly--and that's without a monitor, since I used my old one. If you're talking about a piece-of-shit office PC, then sure, it's cheaper than a high-end smartphone. If you want a PC that can actually run things, then... no.
PS3s are outdated. Compare the price of one at release to the price of the N1 now would be much more sensible. And remember, a PS3 doesn't function by itself. Care to factor in the cost of your HDTV? (Unless you connect your PS3 to thin air and it still lets you play games, I mean.)
Oh, and there're budget smartphones. The N1's not one of them, but even then it's not expensive for what it is.