Looking through the documentations for reCAPTCHA v3, it wants to load a script from Google's servers. As an end user I do not want Google to track me across the web, on other people's property. Thus I want to block this HTTP call. This discussion is pretty orthogonal to "Ad"block, given that all sorts of places have a CAPTCHA implementation.
The cynical side of me thinks that Google's doing this such that a convenient side effect is crippling the user experience for those who uses content blockers/custom user agents to impede its adoption. Even without a cynical motive, this side effect seems to be conveniently ignored, which hurts users. Suppose another non-ad based company were to create such a system, I'd imagine they would want to explicitly work around the problems of blockers/tracking such that the website owner can get a more "true" metric of "suspicious" users as this would likely lead to more sales/engagement/whatever.