Artificial Intelligence (AI) by computers is why CAPTCHA controls have become more complex & difficult as shared in detailed article by Verge below:

https://www.theverge.com/2019/2/1/18205610/google-captcha-ai-robot-human-difficult-artificial-intelligence

Jason Polakis, a computer science professor at the University of Illinois at Chicago, takes personal credit for the recent increase in CAPTCHA difficulty. In 2016, he published a paper in which he used off-the-shelf image recognition tools, including Google’s own reverse image search, to solve Google’s image CAPTCHAs with 70 percent accuracy. Other researchers have broken Google’s audio CAPTCHA challenges using Google’s own audio recognition programs.

Machine learning is now about as good as humans at basic text, image, and voice recognition tasks, Polakis says. In fact, algorithms are probably better at it: “We’re at a point where making it harder for software ends up making it too hard for many people. We need some alternative, but there’s not a concrete plan yet.”

Aaron Malenfant, the engineering lead on Google’s CAPTCHA team, says the move away from Turing tests is meant to sidestep the competition humans keep losing. “As people put more and more investment into machine learning, those sorts of challenges will have to get harder and harder for humans, and that’s particularly why we launched CAPTCHA V3, to get ahead of that curve.” Malenfant says that five to ten years from now, CAPTCHA challenges likely won’t be viable at all. Instead, much of the web will have a constant, secret Turing test running in the background.