Comment by reflexco

Google captchas are hard because they're mostly based on heuristics other than your actual accuracy to the stated challenge. If they can't track who you are based on previous history, it doesn't matter how good you answer, you will fail at least the first few challenges until you get to the version with the squares that take a few seconds to appear. This last step is essentially "proof of work", in that they're still convinced you're a bot, but since they still can't completely block your access to the content, they resign themselves to wasting your time.