Any curve will appear flat if enough magnification is applied. At sea level, a 6 foot tall person will see at most, 0.024% of Earth's curvature. At an airliner's cruising altitude, you will see at most, 1.0% of the curvature, but only from the cockpit. From a passenger window, you will see about 1/3 of that.
Draw a circle on a piece of paper. Then cut out a 1% piece of the circle, and notice how little curvature you can see. This is why it is so difficult to notice Earth's curvature from the surface.