I can see your argument. The fact is that consoles are very limited PCs, and I mean really really limited. Browsing the web on a PS3 is a nightmare. The way that it handles flash is just ridiculous. On that matter, consoles also stink at productivity/utility tasks, which can be easily handled by not-so-limited PCs.
Don't get me wrong, my PS3 is the center of all of my Entertainment needs on my living room, but that's about it; Hence, why consoles are not considered PCs.
I wouldn't call tablets running a mobile OS a PC either, they just don't compare to a full desktop OS in many regards, productivity and utility tasks on them are often complete nightmares in comparison to a Desktop OS.
It's a type of computer, known as a console.
I don't think people will start to call consoles computers anytime soon, whereas people buy tablets to replace computers.
A certain audience who only perform simple tasks could do with just a tablet yes, but for a lot of people something like an iPad or Android tablet couldn't replace a Computer, it's a consumption device meant to compliment a PC. Who honestly would say "Pass me the PC" if they wanted you to hand them a tablet?