Then you'd just give visitors of your websites no recourse and no information whatsoever on how to fix the problem. The benefit of client-side CAPTCHA is that humans at least can pass it and fix the problem even if something they don't control (such as their IP address having bad reputation due to shitty ISP) is causing problems.
As a website operator it's easy to look at the spam that is getting through and be happy that's it's zero. But do you get any idea how many actual humans that you have incorrectly rejected? You don't have that data and it's really easy to screw up there.
Of course if your website is small nobody cares. If you are bigger like Stripe you simply get bad publicity on HN. People on HN love to hate on mysterious bans and blocks just because they do something slightly unusual and your backend-only analysis flags them as suspicious.
As a website operator it's easy to look at the spam that is getting through and be happy that's it's zero. But do you get any idea how many actual humans that you have incorrectly rejected? You don't have that data and it's really easy to screw up there.
Of course if your website is small nobody cares. If you are bigger like Stripe you simply get bad publicity on HN. People on HN love to hate on mysterious bans and blocks just because they do something slightly unusual and your backend-only analysis flags them as suspicious.
Abuse fighting is hard.