I guess another question is why we really need captchas. What are we trying to protect against that can't be accomplished with rate limits, voting systems, or other ways to regulate meaningful use of a website? Ultimately why does it matter if the user is a human or bot, as long as they are being a valuable user? What's wrong if a bot buys some of your inventory, pays for it and everything? What's wrong if an NLP bot…
100% of the time, a bot buying things from a store is doing so to test a database of stolen credit cards the bot's owner has purchased/stolen. Accepting those sales means you'll get hit with chargebacks a few weeks later as the real owners of those cards see their statements. Then your store gets shut down for exceeding the maximum 1% chargeback ratio mandated by Visa and MasterCard. So preventing this scenario matters a lot, and when someone targets one of my stores for testing like this, enabling a CAPTCHA on the payment page is one of several, often-essential mitigations. Blocking IPs, blocking whole countries, including a nonce in the form, etc are on their own insufficient most of the time: the readily-available tools for this kind of attack already handle rotating IPs, retrieving a new form nonce on each try, spoofing the proper referrer, etc.