The better question is "Why should groups willing to split up be able to use the "singles only" line get up the mountain faster than groups that don't split up and waste space at the top of the mountain waiting for their friends?"
Again, this is only about etiquette, not policy. Nobody will call you out on it. Call it a guilty pleasure, but not accepted as totally cool by everyone on the mountain.
We could also make EVERY line a singles line so nobody has any potential advantage. Heck, that's the way it is to get in to any venue, theme park rides, or shopping checkout. A quad would have a bunch of one person wide lines that funnel out to four at the end. Everybody moves at the same pace and everyone has to find their friends at the top.
Not sure I completely understand that hypothetical question you pose. Probably just not reading the wording the way you meant it to sound.
Space has nothing to do with anything, Everyone is in some space somewhere. And people (groups) who split to use single lines end up waiting for each other to. And still they do it because they usually still get up faster which imo just goes show how it is a form of line cutting. And for those who didn't split but were simply too many for one chair and now wait for each other, they are even further apart many times because of the whole singles line game playing thing the others are pulling off. So not only do they get cut in front of but then the second half gets cut in front of causing them to wait even longer to re-group at the top.