That should be the reason, yes. But it seem that for most, the reason is that solo mode is 'easy mode'. Which it shouldn't be. Thre should be enough hard NPCs around that solo mode isn't easy, just devoid of other humans. And we already have NPCs playing the 'homicidal jerk' role that some humans delight in playing, so you don't even avoid those. But because the NPCs are not hard enough (though they seem to be getting tougher), solo mode is easy mode.
Which is why I play open, even though I don't like fighting, and am not very good at it. But I am not stupid enough to fly a T9 to Lave.