Dumb question - why can't I just upvote and downvote pages, and Google enhances its ranking for me based on that?

Maybe that's why the search "X reddit" is so popular - it's got a bit of that social validation.

Google already has similar signals:

1. Do you click on the link in the results page?

2. After clicking on that link, do you go back to the search or not?

3. How long do you stay there before coming back?

Adding an explicit up/downvote would capture some additional information, but it would overlap with the above, and most users would ignore the button. I’d guess Google has at some point experimented with this and found it wasn’t worth keeping.

It obviously doesn't work, otherwise stackoverflow ad-filled copies wouldn't exist.

Can you give me some?

I love stackoverflow content, but hate the surface, e.g. how it loads jquery from google (???) and if I disable it, then the content breaks, because obviously the only way to implement a toggleable div is when you send your users browsing history to google directly..