As was identified, most of the scraping occurs via Selenium. Selenium just automates a real browser, so it'd look like 100% legit traffic with a legit user agent (it defaults to using Firefox)
Think about the goal of a scraper, it needs to actually walk through all the content. That doesn't look like a normal user at all. An individual request might look ok, but in aggregate the pattern of a robot pops out.
https://www.usenix.org/conference/usenixsecurity12/pubcrawl-...