Humans might not be “all the same” but computers are better at chess than any human who’s ever lived or ever will live. Computers can solve captchas far better than any human. In fact, there are browser addons to automatically solve captchas when they ask “are you human” so you don’t even have to deal with them.
When you have a computer mathematically determining whether someone is human or not it’s easier for someone to just use another computer to solve it.
A probation on new users starting new topics would do more to ameliorate the spam problem. Have new users wait 90 days, or 6 months or whatever, plus they have to post regularly beforehand before starting new topics. They have to prove they’re real people, then once they’ve proven that they’re real people they can more privileges.