Why do men and women hop from one ual partner to another almost all the time?

Hi all! I've noticed that people frequently change ual partners. Sometimes it happens in a day, in a week, in a month, but almost always less than 6 months. When I was living at college, I remember a lot of my friends and other people on campus having a new lover every week . No joke! The dorms almost always had used condom wrappers on the floors. hahaha! Why do men and women hop from one ual partner to another almost all the time? Don't they know that it's unhealthy to do that? They could get an STD and wind up pregnant too. Why are people so promiscuous? Seriously! I also see so called "steady, committed couples (yeah right-lol)" and even marriage people cheat on each other. Is it human nature? Just because people don't talk about cheating or being promiscuous doesn't mean it never happens. Any answers would be appreciated. Thanks!