It's my guess that this counters the free recruit janusk in turn 2 (or vice versa). It's not quite the same, but close enough I guess.
Additionally, it would just be another thing to have to program in, for the AI to find a spouse (and then a suitable spouse, and then WHEN to find a spouse, and when to actually get married [if you note, you often cannot marry immediately, even if you find a npc] )... which is just a lot of bother and hoo hah that is simply solved by starting them off married.
It does, however, have the unfortunate side effect of giving them a large head start on having kids - fortunately they don't know how to make effective use of children yet.
On the flipside, however, it has the bad side effect of encouraging a play style where players ought to get married asap to counter the ai advantage, whether or not they want to (or have the means to).
