Because:
1. They are celebrities, they have hair systems, teams of makeup artists, and digital enhancement. Even a lot of paparazzi photos are staged. Never measure hair loss based off celebrity photos, none of it is real.
2. Styling and bad hair days. If I style my hair correctly in perfect conditions, my hair can look thick, almost comically thick (for my age). But when I step out of the shower in bad lighting, I'm still gollum from lord of the rings.