With careful planning, we can easily observe Earth’s curvature from an airliner. To “explain” the observation, flat-Earthers invented the excuse that it is caused by the distortion from the shape of the airplane’s window.
In reality, all camera lenses are curved, too, yet most of these lenses produce normal, rectilinear images. The fact that the shape of the window is curved does not mean looking through the window will produce a curved horizon.
While on the ground, we can easily confirm if straight lines still appear straight through the window. If previously known straight lines remain straight through the window, then a flat horizon will certainly remain straight up there. If the horizon appears curved, then it must be from the shape of the horizon itself.