Only the stars near the horizon twinkle, while those overhead do not twinkle. Why ?

Stars near the horizon appear to twinkle more because their light passes through a thicker layer of the Earth’s atmosphere before reaching our eyes.

The atmosphere contains layers of air with varying temperature and density, which continuously refract the starlight in different directions which causes rapid changes in the apparent brightness and position of the star, producing twinkling.

Stars directly overhead pass through a shorter and more uniform path of the atmosphere, so the effect of refraction is much smaller and they appear steady.