"For distorted text fields, humans took 9-15 seconds with an accuracy of just 50-84 percent. Bots, on the other hand, beat the tests in less than a second with 99.8 percent accuracy."
I'm guessing part of the answer is (and most likely already implemented in things like reCAPTCHA) is rate limiting and detecting bots when they solve these too quickly.
The bots would just slow down then. Their time is ~free.
reCAPTCHA is one of the better captcha because they do a decent amount of browser fingerprinting and their captcha are interactive.
Still there are services for solving them. Fun thing is you only need to pay those services for first ~50K captcha and then you can train your own solver using the data you collected.
Ultimately, captchas only serve to increase the cost of running bots. If what ever your trying to protect is worth more you will fail.
I'm guessing part of the answer is (and most likely already implemented in things like reCAPTCHA) is rate limiting and detecting bots when they solve these too quickly.