Probably because all three companies have different agendas on how to develop their console brands? Like how Microsoft's after the "Entertainment device" thingum. Also there's the divergence in gameplay with 360's Kinect, Sony's PSMove and Wii and Wii U's WiiMote and GamePad. Different companies want to pursue different methods of gaming using different hardware, so it seems a little impossible to reconcile all these... Plus if there's really going to be a "unified" console for Sony, Microsoft and Nintendo at the same time, who's going to actually be in charge for it? And how would the profits be split?
Besides, the industry isn't exactly in a stage whereby manufacturing hardware isn't economically viable anymore, so it's still profitable, so why not?
