Why is reCAPTCHA only used on registration form and not sign-in form?

[Justify]

Why is reCAPTCHA only used on the registration form area and not the sign-in form? If reCAPTCHA was available on the sign-in form would that offer additional security? Is the expectation that if reCAPTCHA prevented a malicious new account, than there is no need to worry about using that in the sign-in form?
Just curious. 🙂 

[Alejandro]

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