There are investment sites accept bitcoin as a payment, but the problem is that most of those investment sites aren't 100% safe, some of them went scam after a few months and bring those bitcoins or cash with them, and most of them after running with money, they will create another website to do the same thing that they've done before, Nowadays some people are using Bitcoin as stock, when bitcoin rate is low they are going to buy bitcoin and once the bitcoin rate increased they will cash it out! That is what other people do a few years ago, I believe a year ago, bitcoin rate is so cheap but nowadays if you have funded bitcoin a year ago, and cash it out today you can even buy a car.