Why does the moon appear fairly flat to a naked eye?

The moon looks flat because it is very rough, and hence is not a perfect Lambertian reflector.

Many dull objects are well described by Lambert's cosine law: the intensity observed from an ideal diffusely reflecting surface is directly proportional to the cosine of the angle $\theta$ between the direction of the incident light and the surface normal ($I=\min(0, I_0 \cos(\hat{l} * \hat{n} ))$ where $\hat{n}$ is the normal vector and $\hat{L}$ the light direction vector).

However, this is a bad approximation for very rough objects. The problem is that the surface is full of facets pointing in different directions, yet we see an average of their light contribution. This means that a patch on the moon near the edge will have some facets pointing straight at the sun and spreading Lambertian light towards us, looking brighter, and a patch right at the centre will have some facets in shadow, looking darker. This can be handled by more elaborate illumination functions like the Oren-Nayar model (more).

There are some further aspects of lunar geology that makes it slightly retroreflective (see also opposition surge), further reducing the contrast between centre-edge. A lot of this is shadow-hiding: when you are looking almost along the lines of sunlight you will not see the shadows cast by objects because they are of course behind the objects and hence obscured to your vision.

Jupiter is presumably significantly flatter than the Moon (and actually reflects light through a different scattering process). Mars is also rather rough and hence flat-looking in telescope pictures.