Why do planets appear as small dots, even with my 10 mm eyepiece?

Planets are small and far enough away that they will never fill a significant portion of your field of view, even at you scope's highest usable magnification.

If you want to see a larger disk, you need to use a higher power eyepiece. Remember that the smaller the focal length eyepiece you use, the higher the magnification you will see in the scope.

For example, many of Celestron’s basic telescopes come with a 10 mm eyepiece as the shortest focal length in the box with the new scope. Using a 5 mm eyepiece instead will double the magnification compared with the 10 mm eyepiece. Your scope will show a larger disk than what you are seeing now. It will still be a small disk, but you will potentially be able to see more detail than with the provided eyepieces.