I'm saying it because it's not only obvious with even a moments thought (you can literally just ask it for an entirely red image or whatever), but also because it's easily provable.
Prompt: "Under the sea"
Image:
Average pixel colour:
Prompt: "a man with red hair wearing a red coat standing in front of a red background"
Image:
Average pixel colour:
So I ask you the same question. Did you just say that because you felt like it was true?
I'd expect that many images are going to be somewhere near 50% grey if you average their luminance out overall. That's just the average of every colour though. The fact that averaging a range of things tends toward a standard distribution isn't particularly surprising. Again though, it's not hard to get a diffusion model to generate something outside of that expectation.
Prompt: "night sky"
Image:
Average colour:
Average brightness: 21%
Prompt: "lineless image of an old man drawn in yellow ink on white background"
Image:
Average colour:
Average brightness: 90%