Although the sound may not be the same as the English sh, I think shh is indeed used in writing in Spanish. Here is one example from the Net.
Tienes que dejar que te explique—comenzó a decir.
—Shh —dijo Dexter, y de nuevo, le rozó el pelo con los labios.
OK...what happens is that I guess, I can only go so far due to my age and i am 62.... and all of these are just suppositions, since I don't know since when we Chileans (I cannot speak for anybody else) have been using that shhh in written form and since when we have producing the sound "a la chilena...chhhh) But I told you "chito" is the word to command silence.
Maybe, movies had to do something about this. You see, all of the movies from Europe or the U.S. were subtitled, mostly I would guess in Mexico. So it is natural to have seen it written in English as it didn't have a translation and nobody would just write it like the sound chhh in the subtitles.
Same thing with a lot of comics that would come imported, again, mainly from Mexico.
Then again, we are a melting pot in Chile and the usual big British corporations got hold of the economy and of course of the main business language.
Many British and American companies, that would bring their top people and they interacted with the general population starting with their Chilean workers...
Many words were extracted from there and incorporated into our castilian.
Same thing with Football.
Again, I don't know how far back this goes and if before all that interaction with the British we were saying chhh or shhh or whatever.
I am hoping for comments on all this...there must be millions of reasons, but somehow I think more or less the Hispanic world as been subjected to the same experience, one way or another.