> All I remember is the default green gradient thingie and…

IIRC at that time users were split fairly completely into three types: those who customised their OS look a lot, sometimes spending far too much time on it (I was one of them back then), those who had developed a preferred look and just kept that as much as possible between OS changes, and those who just used the defaults and got on with whatever else. This means the other wallpaper that someone at MS put effort into collecting together hardly ever got seen: many users kept the one default, and almost all who didn't had something that they chose from other sources. Maybe the other included images might have seen the light of day more often if there had been an explicit “choose your wallpaper” prompt as part of user on-boarding.