Scrapers are usually coded to have as regular of a usage pattern as possible, so that the data they retrieve is as much like the data the end user would receive as possible.
For example, Googlebot does everything in its power to ensure that it sees pages the same way that end users sees them, executing JavaScript and performing OCR to try to read information conveyed in images. Google also has non-Googlebot scans to try to determine if a page is serving different content to Googlebot-labeled scans, and they penalize sites that they suspect of doing this.
While it is true that someone could write a scraper that obviously behaved robotically, it is also true that someone could use their desktop browser in a robotic way. Mashing F5 is so common that there are many ancient memes referring to and making jokes about that activity. There are extensions that end users use to record browser macros, behaviors they want their browser to repeat over and over again.
However, this conversation about whether scrapers behave robotically or not is moot because a web site shouldn't break down under load when someone uses it in a slightly-irregular way. The obvious, crappy scrapers are trivial to block. The ones that blend into the traffic are no harm, no foul. If you can't tell the difference between an optimized browser like a scraper and a general-purpose browser like Chrome, why shouldn't it be allowed to talk to your site?
