Why can’t a bot tick the ‘I’m not a robot’ box?

For starters, Google invented an entire virtual machine – essentially a simulated computer inside a computer – just to run that checkbox. When you click that checkbox, Google reviews your browser history to see if it looks convincingly human. Your bot might have to sign up for a Google service and use it convincingly on a single computer, which should look differently from the computers of other bots, in ways you don’t understand.

Source: www.quora.com