I'm pretty sure some insects have brain functions in their torsos or knees or whatnot, but evolution didn't branch that way for any of the mammals. Must be some pressure not to.
You do get the odd "You can't get there from here" moments in evolution. It'd be a pretty major restructuring to have the brain migrate to parts south (or, in my brother's case, parts north), and I'm not convinced the intermediary steps would be particularly effective (eg, having your brain in your neck). Even if there were a significant advantage, I doubt it would happen.
Incidentally, I'm thinking that one consideration here is the brain needs to be very firmly encased, which means a solid bone covering (though the recent discovery of the world's smallest fish - was it tommyrot linked to it? - doesn't have a skull, as I recall), while most creatures want pretty flexible torsos to make high-speed travel easier. That could suggest an advantage in the current arrangement.
Same pressure that gave us the knee? I figure sometimes stuff just works well enough to not have the species die out. Get us as far as sapience, and we'll start picking up the slack.
Yeah, pretty much. The knee isn't really very well designed for bipedal living. Pops too easily. But it's a leftover from our quadrupedal heritage, like the plumbing arrangements that make us susceptible to hernias. But as long as the bipedal stance offers more advantages, we live with the drawbacks. Like Kriss Kross wearing their jeans backwards.