Yeah, it's odd, bikers hold their heads level in corners but lean their bikes and bodies while in a car I tend to lean my head towards the inside of the curve. I wonder if both are for the same reason, something to do with balance perhaps.
No..on a bike, you are looking well ahead..way far ahead through the corner. And since modern bikes on modern rubber can lean beyond 50 degrees...you wouldn't be able to look very far ahead if your head was tilted as much as the bike..
Yeah, I guess if you didn't tilt your head so it was more or less level and turned towards the curve then you'd be trying to tilt your head "up", which doesn't really work as well.
I've heard a few different arguments over the years as to why it's done, I dunno, I just know everyone does it.