Why do people suggest not to do this anyways?

I'm 23 yr old guy. Some people suggest not to have sex with random people. What is the reason behind that? STD's? You can always have them tested first before having sex with that person. The awkwardness of meeting them afterwards? How can that be awkward to meet them again after having sex with that person? As they grow older, why do people choose only one partner and stay with that person for the rest of their life? Is it worth it to have sex with many people in a lifetime vs only one person that you have fallen in love with?