Answer:
Step-by-step explanation:
Basically because they are big and relatively near.
The light coming from a star travels a huge distance and is acted upon by the earth's atmosphere with causes all sorts of shifts. What we are seeing twinkle is caused by these shifts, for the most part caused by atmospheric density and heat.
The beam from a planet is not narrow enough to be affected the way the stars are.. If you were out in space looking at all this, the stars would not appear to twinkle either.