To avoid DDoS and content scrapers that would harvest the article text and put it on link farms to get clicks from confused search engines.

Because there are a lot of bots out there. Some are neutral scrapers just contributing to website load, while some are actively malicious. CAPTCHA's were a crucial development that allow the internet to continue to function efficiently and openly (for humans) as internet access expands in regions where there are no repercussions for cybercrime.