Even in the case where you're getting some kind of behavior or reputation verdict from past behavior (and possibly across multiple surfaces), you probably want a progressive set of outcomes rather than just a binary allow or deny. Even if some requests are clearly best just blocked and others should obviously be allowed, there's always going to be a grey area where you're not sure. You need something to do with those requests. Making an arbitrary choice is one option, but pretty harsh on the legit users. A captcha in another.
Sometimes you have options for that gray area that are much better than captchas that you can do, e.g. request a phone number and do an SMS challenge. But that's both expensive and will lead to a massive dropoff for most sites as people won't be willing to give out their phone number to every site.
(Also, the act of solving the puzzle can give you additional signals of whether the request is from a bot or not. Signal collection is kind of the entire point of the slider captchas in the first place.)
