The build new bots to beat the bot checkers.
Every generation of captcha is designed to prey on a flaw in the current generation of ai and pick a task that the ai cannot do but humans can.
The problem is that as soon as the bot designers can see what it is that the captcha are using to stop the bots, they can start designing new bots that are able to defeat those.
It turns into an endless game of cat and mouse, with each group constantly leapfrogging the other.
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