Why do some elements with high electronegativity exhibit metallic properties?
Whether an element adopts a metallic or nonmetallic structure does not depend on electronegativity alone.  Aside from electronegativity at least two factors promote formation of a nonmetallic structure:

1)  If the atoms are close to filling their valence shells they are more likely to do so through covalent bonds.  Iodine needs only one more valence electron per atom and can get it by forming covalently bound diatomic molecules; gold needs seven valence electron per atoms to complete the valence shell and goes the metallic bonding route instead.

2)  elements in earlier periods tend to form stronger covalent bonds favoring the nonmetallic structure.  Silicon form the familiar tetrahedral semiconducting structure but lead, below it with similar electronegativity and valence electron configuration, is metallic.

Other factors may enter, too, and Group 14 is an excellent place to look for subtle effects.  Above I said that silicon has the tetrahedral structure we all know and love, but that is under ambient conditions. The tetrahedral structure of silicon is energetically more stable than the metallic one, but the latter has more entropy and thus silicon may be converted to a metal upon heating.  In the case of silicon the temperature for this switchover is high enough so that the metal is molten, so we have simultaneous metallization with melting -- and, therefore, contraction with melting.  Germanium does the same thing, whereas with lead the metallic structure is more stable all the time.  Tin is a transitional case where it's a semiconductor when cold but switches to a metal below the melting point.

So there is indeed more to what makes a metallic element or a nonmetallic one than just electronegativity.