Because you can totally work the systems to get it affordably. Besides insurance, there are clinics, there are coupons, there are programs…but you have to have the information literacy and the time to figure out how to jump through all those hoops.
Me, I work remotely, I have insurance, I live on the north side of Chicago, I can get my Truvada easily. You live on the west side of Chicago with no Howard Brown nearby, and you work a service sector job where your hours are algorithmically doled out to you? Far, far harder.
This is true of many medications, but Truvada is especially famous for it. As a gay man I have a lot of friends on it, and because of the aforementioned program, only two of the thirty+ pay any co-pay at all, and it's something like 20 bucks for them.
I don’t know anyone paying full price for Truvada. It’s either free, or whatever your insurance copay for drugs happens to be. (It’s annoying I have to go to a specific pharmacy with my drug plan, but whatever.)