Look a little closer
Stars twinkle while planets usually look steadier because of Earth's atmosphere and their apparent sizes. A star reaches us as an unresolved point of light, whereas even a tiny planetary disk combines many points whose fluctuations tend to average out.
The atmosphere is not a uniform transparent sheet. Moving pockets of air with slightly different temperatures and densities bend light by changing amounts. Their constantly shifting focus and path produce atmospheric scintillation.
A star may be enormous, but its distance makes it nearly point-like to the eye and most telescopes. As that narrow beam passes through changing air cells, its apparent brightness and position fluctuate rapidly; wavelength-dependent bending can also create hints of changing color.
Planets are much closer. Although they may look like points without magnification, they subtend small disks that can be larger than the atmospheric fluctuation pattern. Light from different parts of a disk crosses different air paths, so a dimming patch is partly balanced by the rest.
It is therefore too absolute to say that planets never twinkle. A planet with a small apparent disk, especially near the horizon, can scintillate after crossing a longer and more turbulent atmospheric path. Stars also tend to twinkle less when they are high overhead.
Above the atmosphere this kind of twinkling disappears. Ground observatories choose high, dry sites and use adaptive optics to reshape mirrors against rapidly changing distortion. The familiar shimmer is not a star trembling; it is a visible trace of moving air above us.
