Tor: Why do I see so many CAPTCHAs on Tor?

Short answer

Many people share the same Tor exit IP address, so a website's fraud system may interpret the combined traffic as automated or suspicious. A CAPTCHA does not mean your device has spyware. You can try a new circuit for that site or wait and retry. Repeated prompts may remain because the site's policy applies broadly to Tor exits.

Sources

Tor Project: CAPTCHAs.