Why do people say like "down the hall, road, alley, etc" when it is not down in any way. Why can't we say like "it's straight this corridor, road, etc"?. Why do say "it's straight down this corridor, road, etc"?
Please elaborate it.
"down" as preposition or adverb does not always mean at/to a lower point. With street or road it simply means along as in: The bus stop is a bit further down on the left side.
In this case of the sentence, down is being used as a preposition. Down as a preposition has two main meanings:
From the higher end to the lower of.
From one end to another of.
While it is derived from an Old English word meaning "Hill", I believe that common use hundreds of years ago led it to have this second main meaning.
To address your point about saying "Straight this corridor." Straight is not a preposition. It is grammatically incorrect to say "I am going straight this corridor." "This corridor" has no place in the sentence, since "straight" is an adjective and "this corridor" is not connected to the predicate or subject.
"Downstream" does mean the direction with a (slight) downward vertical component; it's possible that the idiomatic uses of down (downwind, down the street, downtown) come from that sense, all meaning "in the natural or most important direction of travel", i.e. "with the current".