What does “free” mean? You might be asking why the metallic bond formed. The electrons appear to have been delocalised across the entire structure and can be shunted with a trivial electric field difference.
Let’s say that around an atom, valence electrons decide they want to pair up with similar electrons from another atom. Although they might appear to form covalent bonds, it is difficult to see how. The electrons can be a simple linear combination of waves and electrons from another atom. This does not make a strong bond. Because it is so small, hydrogen is an exception. Let’s say you have three atoms along a line. Now, think about the centre one and imagine it sharing an electron with its right-hand neighbour. What can you do to stop the electron wave function of the left-hand neighbour interfering with your centre one? The centre electron can bond equally with any neighbour if this is the case. You can extend this analogy and see that electrons are more likely to be delocalized if there is no strong covalent bond. This is because each overlap is basically the same. Because all electrons behave the same and there is regular spacing among the atoms, it’s possible to talk about an “electron ocean”. However, the electrons aren’t “free” as the usual meaning of that word. They are bound in a different manner.
Non-metals favor covalent bonds because p waves are better at concentrating charge and making stronger covalent bonds. This is why the energy barrier to getting them to “switch atoms” is higher. Structure is also crucial. The structure is also important. In diamond, for example, the wave must “turn a corner” in order to enter another wave structure with an atom. This means that the wave must be refracted. This is why d electrons and electrons are better for it. It’s easier for them to move into the new arrangement.
Sometimes people will say it is easier to remove electrons from metals. However, this is generally false. This is because mercury has an ionization potential of 10.4 eV. Zinc has an IP of 9.4. Important is the ability of covalent bonds to move. Diamond is therefore an insulator, graphite conducts as the covalent electrons in graphite are delocalised.