What causes websites to block web scrapers?

Websites block web scrapers mainly to protect their resources, data, and business interests. Common reasons include:

• Preventing server overload: Scrapers can send many rapid requests, which can slow down or disrupt the site for real users.
• Protecting proprietary data: Some sites don’t want their content copied, republished, or sold by others.
• Stopping abuse and fraud: Scraping is often used for spam, price manipulation, account attacks, or mass harvesting of user data.
• Preserving competitive advantage: E-commerce, travel, and marketplace sites may block scraping to stop competitors from collecting prices, inventory, or listings.
• Reducing legal and compliance risk: Sites may need to limit automated access to avoid violating privacy rules, contracts, or terms of service.
• Defending against bots: Some scraping is part of broader bot activity, so sites deploy anti-bot systems that catch both harmful bots and legitimate scrapers.

Common blocking methods include rate limiting, CAPTCHAs, login requirements, IP blocking, user-agent filtering, and more advanced fingerprinting or JavaScript challenges.

If you want, I can also explain how sites detect scrapers or how to scrape ethically and legally.