What kind of planes are you flying?

I mean, I just checked, it seems under ideal conditions one could see the curvature of earth at 10.5 km height, but to me it was not really a convincing curvature last time I did a long flight. Your experience was different?

10.5km (34.4kft) is an ordinary cruising altitude for an ordinary commercial airliner.

The curvature of the earth can be quite apparent on a clear day at that altitude when flying over water. Or the midwestern US.