Compared to rocks, ice is not that dense. You can try yourself: it's easy to pickup a big chunk of ice, even with a shovel, while a similarly sized rock is impossible to lift.
"Other substances that expand on freezing are acetic acid, silicon, gallium,[35] germanium, bismuth, plutonium and also chemical compounds that form spacious crystal lattices with tetrahedral coordination."[0]
There's more than water. A lot of things that freeze into a crystal lattice can be less dense than their liquid states. Silicon dioxide is another IIRC.
There is a very wide range of snow. In particular, the heavy wet stuff acts and behaves very differently from "dry" snow -- whether that's light powder on top, or compacted, but dry, snow underneath.
Wet snow is increadibly heavy, and frequently contains or mixes with ice which makes removal (blowing, ploughing, shoveling, 'dozing) far more challenging.
