Most of us have flown in a plane @ 30,000 ft. or so and know your statement to be false.
You can't detect curvature at that height (and I have flown plenty, my work involves travel).
But you can measure the horizon dip at much lower altitudes, several experiments have been outlined and shown on here recently which do that.
Even if NASA are faking all their images (which they aren't), there are plenty of space agencies which have taken film and pictures from space and even amateurs have sent balloons up high enough to see the curve.