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.

[0] https://uifaces.co/

[1] https://avatars3.githubusercontent.com/u/383701?s=460&v=4