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They say AI is advancing greatly but I'm not convinced, most bots will still be bound to automated scripts and google should be able to avoid that. I think they also want to avoid inconvenience as its a standardised system
I'm not convinced that anything as sophisticated as AI is required for reCAPTCHA, I've been told they break it with good old fashioned brute force. Even with that being the case it is still the best captcha available at the moment.
In some instances it is actually too good, because it beats me some times.
In more than one instance, I had to re-do the Captcha 2 or 3 times, because it wants me to skip it and then re-do it again. I also get some funny things with it, when I use it over a link with low bandwidth. 
I can see how AI would struggle with solving reCAPTCHA, so this is a good bet to use them.