Why do metals form ions in solution?
Solids can't have a charge (neither can liquids or gases). The electrostatic forces are immensely great, way greater than you seem to think right now. Just imagine the forces that would be tearing apart a droplet of water (say, 36 microliters) if 0.001 of its molecules (not all of them, God forbid) would suddenly become positive ions. If you are aware of the Coulomb's law, you can help your imagination by actually calculating these forces.

(By now you may wonder how electrical phenomena are possible at all, but that's another story.)

Just in case if you don't like that answer, here is another:

Solid zinc, like any other metal, does have a charge. It consists of $\mathrm{Zn}^{2+}$ ions. These ions are immersed in a sea of electrons, which balances their charge, holds them together, and also is responsible for such properties typical of all metals as electrical conductivity, metallic lustre, etc.