The question is in the title. Simply put, I see no reason why "surfing" in particular was chosen to describe…whatever it is we are doing when partaking in this gigantic network. I can't see any good analogues in this activity to surfing terms either. I see no surfboards (except maybe ipads and similar flat devices), I see no high tides, and there is a distinct lack of surfing terms such as "gnarly", "goofy", "hang loose" and "grommet" in the current usage of Internet lingo (although we have many other Internet-only terms instead to cope with this severely limiting shortage of vocabulary :) ).
So why do we call it "surfing the web"?