Because, like large swaths of Southern Nevada, the airspace over Area 51 is restricted. Area 51 falls in restricted airspace R-4808N, and it's not legal to overfly it without permission.

https://www.faa.gov/documentLibrary/media/Order/2022-02-16_O... (search for R-4808N)

https://skyvector.com/?ll=37.22343947130071,-115.74522399506...