Planets are a lot brighter.

The “twinkling” of Stars is a matter of distortion of the light as it passes through the atmosphere. Because they are so far away, the beam of light is much narrower than the light reflected off of a planet. The apparent diameter of a planet relative to a Star is much greater (although stars are many orders of magnitude larger than planets).

Stars do not twinkle when observed from outer space.

Distant planets do appear to twinkle when observed from Earth.

Planets don’t have their own light source. They reflect light from other stars. That’s why they appear brighter.

Stars emit their own light. Atmospheric interference between us and the stars makes them appear to twinkle.

The image of planets is larger, because they are much closer. A slight apparent change in position due to atmospheric distortion is not noticeable, because it is small compared to the size of the image . Star images are like point sources. A slight apparent change in position is very noticeable.
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@LostInParadise is the most correct. My answer at the beginning was lazy (it was sort of correct, but it was lazy).