I think this is because porn is all about making you feel some lust for the lady. And lust is a very practical and specific kind of feeling. It's when you want suck the lady's tit and nail her with your cock.
So, you need to look at it from a practical point of view to understand why one tit works better than two tits.
You have only one mouth to suck the lady's tits with. And it can be hard for you to decide which tit to suck, when the lady presents to you both of her tits at the same time. Your feeling of indecision competes with your feeling of lust and makes the whole thing less hot for you.
But when the lady clearly presents only one tit to you. Then this a one-to-one relationship between your mouth and her tit. You just go for her tit and suck on it. There are no other interfering feelings in this situation. It's pure lust. And that's why it works better in terms of hotness.
The same can be said about making you want to nail the hot chick with your dick. In a DP for example, both guys banging the lady hard and fast make it hard for you to decide how you want to ride.
In addition to your lust, you end up feeling a little confused. Do you want to be her ass-guy, or pussy-guy? You have to choose, because you have only one cock. Here is some examples:
But the DP pictures shown below are hotter and produce better lust. Because one guy is clearly having more fun than the other. Which makes it easy for you to decide how you want to ride. Your feeling of lust in this situation is pure without any other interfering feelings.
Here you want to be the lady's ass-guy:
And here you want to be the lady's pussy-guy:
