Why does the APA/MLA format suggest the use of double-spacing?
You need to separate authoring the manuscript from the finished type-set layout of the journal. For manuscripts more loosely spaced single column format has been the norm. This is because before the digital age, manuscripts were handed in written on a type writer. There was no means for an author to produce sophisticated layouts. In addition, one would add comments manually in the manuscript which is why some journals also specify fiarly wide margins. The marked-up manuscript would then be sent by regular post to the editor and passed on to the author. The one column loosely spaced format has stayed although it is not necessary. That said, reading a loosely spaced single column manuscript is easier than a densely set double column format. The latter is of course more cost effective to print. 

Many journals use (or at least allow) LaTeX templates for their submission. These commonly include settings for a "manuscript mode" which end up as single column, loosely spaced text but also a "layout mode" where the author can try the final format of the journal to assess the length of the finished paper. This also allows the journal to more or less automatically type set the final paper once it is accepted.

So the recommendation for manuscripts to be typed loosely spaced and single column, is partly tradition but also because that format is easier to read for the reviewer.