By that logic, it's also possible to avoid high grocery bills by sticking some things in your pocket and not paying for them. Just because you can do something doesn't mean it's a scalable or ethical option.
Google's core breakthrough in advertising was to close the loop and push advertisers towards ads that are relevant to customers, where they know exactly how much they're earning per dollar they spend on ads. Ads have, actually, improved quite a bit over the past 10 years. The famous line was "half of ad spending is wasted, we just don't know which half" (paraphrased)... now they know which half.
To go back to the OP's subject - the reason why retargeting has grown so popular is that it WORKS LIKE A MOFO. Retargeted ads are usually an order of magnitude more relevant and are immensely cost effective in terms of spend versus conversions. The best indicator that someone's interested in buying a Ford? They visited the Ford site.