Why hasn't Google implemented this yet? Their current solution isn't good enough (https://developers.google.com/webmasters/ajax-crawling/).
You are right that it is not a real-time exercise but they do have crawl targets.
You cannot be flippant about "Why don't they just do X" when scale is that big.
[1] Picked out of the air but probably in the right magnitude (or even a little small)
After all, the bot doesn't get impatient.
Some how we all end up antagonistic over bullshit like whether google have a big enough computer.
But alas, you're right, google could never crawl with an actual browser - what a ridiculous suggestion. I apoligise for such a dumb-witted comment.
As an aside: For my part in contributing such bad quality comments, I apoligise.
Anything that significantly adds to the load will lose them money - whether or not the operation needs to be realtime is secondary to that.
I apologise for giving offense: I wrote the comment the same way I would have made it face-to-face, which is always a bit risky in a purely textual medium.
Google executes JavaScript and renders the full DOM for every page internally. They generate full length screenshots of every page and have pointers to where text appears on the page so they can do highlighting of phrases within the screenshot.
It isn't even a debatable question if Google reuses the Chrome engine to do this.