In Spanish for example you would only say "en la calle" - also literally "in". For German it would be "auf der Straße" - "on". I guess each language has evolved a preferred way to express this, and for Esperanto it happens to be "en" = "in" (= "on") ... Oh dear I'm getting confused again ! :-p
I've never really given it much thought before but it's interesting that in English we use in the street to refer to being actually positioned there and on the street for more general, less tangible uses (i.e. "have you heard the word on the street?", "she makes her money out on the street").