Interesting! I can infer now the logic. If the expression was only to use 人山 or 人海, it could not be understood as "a lot of people" because the immediate relation between the terms would be that of description, and since places are used for description the terms thus would come to mean "belonging". That is, 人山 would mean "mountain people" as in "people of the mountains", and 人海 would mean "sea people" as in "people of the sea". But by coupling both the association to the places is broken and can now only be that of what the mountain and the sea have in common which is the notion of vastness. So the literal description comes to be "vast people", which can only mean "a lot of people". Critical is the use of 海 which leads to the idea of "vastness", for other word coupled with 山 could lead to the idea of "size", which can be then used to imply that the people one is referring to are of formidable size or "formidable" in itself.

A pleasure to the mind as always sensei, thank you and I await for the next :)