I could not see the curvature of the earth while I was at approx. 25,000 meters?

I was the passenger of a T-38 a few days ago. I expected to see the curvature of the earth when we went high above the clouds -- however, I saw no curvature, it was a flat line, horizon.

Why is that? I have seen the curvature of the earth when I viewed a GoPro movie of Felix doing his parachute stunt, but I went higher than that and using my own eyes, saw no curvature at all.

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