Most (if not all) default browsers on mobile, auto scroll/hide the navigation bar off the display after a few seconds. For most non-techie users this means it's 'gone'. Ergo, they have no back button anymore. I've watched many people just open/shut/open/shut their mobile browser just to get back to previous pages - in many cases even re-searching the same query over and over again.

This is modus operandi for many users, hence why Google inserting a big 'go back to Google' banner at the top, is a bad[1] thing.

--

[1] Or from the standard user point of view, a good thing.