Some people willingly submit photos to be used in this manner[0], which will lead to them being used all over the place.
Once you've seen this picture of Brad Frost[1] (a web tech personality), it's hilarious how many startup landing page mockups he appears in. I've personally seen dozens.
Just because a testimonial image can be found elsewhere on the web doesn't mean it's fake. The images are of the actual person who gave the testimonial but I changed their names. I see absolutely nothing wrong with doing that.
Once you've seen this picture of Brad Frost[1] (a web tech personality), it's hilarious how many startup landing page mockups he appears in. I've personally seen dozens.
[0] https://uifaces.co/
[1] https://avatars3.githubusercontent.com/u/383701?s=460&v=4