Despite "shockingly low" rates of PreP, transmission rates are still relatively low because more of the people who DO have HIV get treatment and don't spread it like they used to.
40 years ago being gay was more or less scandalous and there were well over 100,000 new cases a year in the US.. Now the MSM population has at LEAST doubled and there are about 30,000 new cases a year. When you consider how many unintended pregnancies happen each year, it's not really like people are so much.worse than they used to be.
Also, HIV isn't something that is spontaneously generated by two men rubbing together. If a closed group of people is all barebacking and none of them is HIV+ and untreated to begin with, nothing is going to happen.
I do wonder if the numbers have been run on just testing everyone quarterly and getting them treated immediately and what that would do to new case rates versus the cost of PreP.