Dude, that's a lie lol. I have as many white friends who date white guys as I do who date non-white guys. Maybe everyone likes everyone now? It's really none of your business anyway besides you making this up XD
I think it is because they like to feel rebellious, though not necessarily too rebellious. Generally as a Mexican American I've had good luck with such girls because I'm also Republican and still pretty damn traditional. So I suppose the girls get their "exotic" fix from me while still keeping their conservative families happy since I at least share similar values as them despite being brown.