There is no need to destroy privacy, waste resources, create terrible ux, gate content…
Things that work for small sites fall apart once you get 10~100 million+ people.
Look, I help do the actual user moderation for a large (10m+) subreddit, and it is a god damn nightmare. Plus, on top of that, we have to do automated bot detection anyways, because otherwise it's just neverending bot spam.
Reddit is actually a good example of having millions of users with no captcha. They “fixed” their problem with user moderation.
You chose to become a moderator, so it must not be that much of a nightmare.
EDIT: Reddit does have a captcha now. My guess is it came along with the other users hostile changes in the new design.
edit: They have for at least the last 6 years based on online questions. Long before the new design.
Automation detection and privacy are orthogonal. The ideal bot detector yields a single bit of information: bot or not. It doesn't need to try to de-anonymize users or anything like that. As for UX, well, spam is a bad UX.
The incentive is money. You can't just wave a magic wand and get rid of the incentive to spam things. Asserting that you can is wishful thinking.