ColourOfSpring - 7:35 pm on Apr 6, 2013 (gmt 0) [edited by: ColourOfSpring at 7:43 pm (utc) on Apr 6, 2013]
Because they ARE different, and that pre-dates Google. If they treated them the same, they'd be breaking the standard.
1script already noted that, as I did - it's obvious. Everyone knows this. However, in actual reality, the www subdomain is an alias for the domain name in almost every single case. As I mentioned in my comment, the fact that Google treat them as technically different (and they are not technically wrong to treat them technically different), it's encouraged so many domain name owners to alternate between the two in the hopes that Google do indeed treat the switch as a move to a "new domain" (which it actually is, but I thought I'd use quotation marks because we all know it isn't really).
In real terms, it's just a mistake to confuse www and non-www in almost every single case because they are aliases for each other. I'm SURE Google (in reality) knows this, but officially doesn't recognise this.
Seriously, anyone who has DNS control over a domain name - they're going to make two different sites - one www, one non-subdomain (non www) - two different sites? Technically they can do that, but it would be a marketing disaster. Who is going to say "www.foo.com" is our site that sells A, foo.com sells B". Nobody does that.
[edited by: ColourOfSpring at 7:43 pm (utc) on Apr 6, 2013]