Why are metallic compounds insoluble in water?

The ability to be dissolved or the "solubility" of a substance really depend on two things: The polarity of the (water) and the amount of energy it takes to break the intermolecular forces of the (the metal or other substance). Because it takes a lot of energy to break metallic bonds, metal is insoluble in water.

Water molecules are polar, meaning they have a small, unbalanced charge associated with the atoms (H's and O) due to unfair sharing of electrons. Oxygen has the capability to tug more on the shared electrons (-) between it and hydrogen causing oxygen to have a partially negative charge and hydrogen to have a partially positive charge. We say a molecule that has this type of opposite partial charges has a dipole (two poles; + and -) and we say that the molecule itself is polar.

Solubility basically works on the concept of attraction. Because water molecules essentially have ends that are charged the molecules can attract other charged particles, attack, and rip partner molecules off of one another. Of course this attack would only work if the weak partial charge attractions from water molecules were stronger then the forces that hold molecules with other molecules (the intermolecular forces). Table salt and sugar have weak intermolecular forces and are soluble in water.

Metallic bonds are held together by very strong intermolecular forces. You can imagine all the nuclei of the metal atoms sitting really close together and freely sharing their elections with one another almost like electrons are flowing around them like particles in a sea. (incidentally this free flowing ability of elections allow metals to be great conductors of electricity). Polar water bonds do not have the strength to remove these intermolecular attractions and therefore, metals are insoluble in water.

Of course, however, anything breaks down over time, but we can thank for that.