I think that's because the particles are all vibrating in different, random,
unrelated directions, with different speeds, so when you add it all up,
it adds up to zero.
If for some weird reason all or most of the particles suddenly moved in
the same direction, then the rock could suddenly lift up into the air.
There IS a chance of that happening, but the chance is so tiny that
even the most careful scientists just assume that there's no chance
at all, and so far, they have never been wrong.
