The captcha itself (matching pictures to text) is mostly for ML training data. I think pass/fail is mostly based on heuristics like how you moved your mouse which could get analyzed before you complete the captcha. https://www.techradar.com/news/captcha-if-you-can-how-youve-...

reason why is 1. Google and others really needed the training data, and 2. it probably helped justify the cost of providing the captcha service for free worldwide (old free tier was 1M/mo)