

Most often because they don’t download any of the css of external js files from the pages they scrape. But there are a lot of other patterns you can detect once you have their traffic logs loaded in a time series database. I used an ELK stack back in the day.
My experience is with systems that handle nearly 1000 pageviews per second. We did use a spread of haproxy servers to handle routing and SNI, but they were being fed offender lists by external analysis tools (built in-house).