I don't claim that this explanation is an original insight, since it's the kind of thing you can figure out just by thinking it over -- as I did today while bored. With that said, here's one reason why: a hot girlfriend is better at sending the signal that her boyfriend is a good catch, so that female strangers are more likely to notice him and decide to make a move on him. Even if the boyfriends of hot girls weren't any more special than those of sub-hot girls, and even if men didn't decide to seek out strangers for increased variety, the hot girlfriend would still bring about her own betrayal more easily than would a sub-hot girlfriend.
Let's say we have a Girlfriend, Boyfriend, and a female Stranger who may or may not sleep with the Boyfriend on the sly. I'm assuming that the affair goes on even though the Stranger knows that the Boyfriend has a Girlfriend, so I'm not talking about cases where the Boyfriend lies to the Stranger about his relationship status. Let's look at a diagram of the states the Stranger can be in:
If she is in state I, she is ignorant of the Boyfriend and Girlfriend's relationship; in state A, she is aware that they are a couple; and in state C, she intends to participate in cheating with the Boyfriend. (I assume that if she makes her intentions known, the Boyfriend will certainly cheat with her, at some time or other.) Think of it like contracting an infectious disease. The Stranger can get exposed to info about their status, but her mind has to absorb or contract that info, rather than it not registering. Once she's infected with that info, she has to move to the advanced stage of the disease where she intends to cheat.
P1 is the probability that the Stranger registers their relationship status when exposed to info about it (and 1 - P1 the probability that the info doesn't register). She could become aware by hearing about their status through an acquaintance, or see the couple out together doing "couple things." P2 is the probability that, once aware of their status, she decides to cheat (and 1 - P2 the probability that she remains aware but doesn't want to cheat).
I'm assuming that if she goes through one of the loops, rather than follow the arrow forward, she stays put there forever. The idea is that, if she is exposed to info about their relationship status but doesn't even take notice, he is not attractive enough for her to ever notice -- girls notice whether desirable men are taken or not, at the first signal. Similarly, once she knows they are a couple, if he isn't desirable enough to move her to cheat, he never will move her to do so.
So, in order for the Stranger to cheat, she needs to go straight from I to A to C, and this happens with probability P1 * P2. Once she cheats, she is forever a cheater, since she cannot undo her decision to cheat, nor can she reverse her awareness of the couple's status.
When the Girlfriend is hot, this increases P1: female Strangers are more likely to register that a couple is going out in this case since hot females end up with highly desirable males, and Strangers will use this as a cue to find out who the good catches are. Moreover, when the Girlfriend is hot, this increases P2: if a Stranger is aware of the status of two couples, she is more likely to decide to cheat with the Boyfriend who has a hotter Girlfriend, for the same reason as before (he's likely a better catch).
Let's compare the two cases by saying that the Boyfriend of a hot Girlfriend will be viewed as an alpha, and the one of a sub-hot Girlfriend will be viewed as a beta. I'll attach the letters "a" and "b" to indicate which case it is.
Well, the total number of extra-pair partners that the alpha will have is:
P1a * P2a * Na
Where N is the total number of Strangers who are exposed to info about the couple's status. And for the beta, his total number of cheating partners will be:
P1b * P2b * Nb
Given the above description, P1a is greater than P1b, and P2a is greater than P2b. So, if the same number of Strangers are exposed to the couple's status, the alpha Boyfriend will have more cheating partners. For example, let's say that having a hot Girlfriend increases both P1 and P2 by a factor of 10 -- then having a hot Girlfriend will result in 100 times as many cheating partners as having a sub-hot Girlfriend.
What would the beta Boyfriend have to do to get the same number of cheating partners? He would have to ensure that (P1a * P2a) / (P1b * P2b) times as many Strangers were exposed to his status. Using the same example numbers as before, the beta will have to make sure that 100 times as many Strangers are exposed to his status. Realistically, you figure the hot Girlfriend and her Boyfriend are already out and about so much, that it would be impossible for the beta Boyfriend to get the word out to 100 times as many Strangers, short of embarking on a world tour.
So we see that, even if all the other potential causes of hot girls getting cheated on were to vanish, they themselves would continue to be the cause of their own betrayal. As advice to hot females, the only way to prevent this dynamic would be for her to try to keep her relationship a secret -- to not tell anyone, and definitely to never be seen in public together doing "couple things." The Boyfriend might resist and try to signal their status, but she could explicitly enter the relationship by saying, "I'll only be your girlfriend if you help me keep it a secret."
Finally, there is some practical, amoral advice for the guys too: if the passion in your relationship dries up, and she is good-looking, you wouldn't necessarily have to get into staged fights or try to make her jealous in order to make her feel insecure, which in turn would cause her to give it to you like never before, to ensure you'll stick around. You could simply let the relationship fade into asexuality while parading her around to attract strangers, some of whom will make a move on you.
This tolerated adultery is the norm among European politicians and their wives, while in the US we panic and search for methods to keep the fire lit between just the original two -- introducing whips and handcuffs, for example. Notice that all varieties of kinky practices take place among established couples typically over the age of 30, never among college students whose hormone levels are high enough by nature that they don't need an artificial injection of kink. Doing it while the parents are home is about as oddball as it gets. To each their own in resolving the problem, but I'd probably choose tolerated adultery.