In zombie films, how come nobody has ever...
..put on a suit of medieval full-plate knight armour?
Instant, near-total invulnerability to zombie bites and scratches.
A squad of plate armoured knights with sharp swords and shields could easily cleave through a zombie horde, decapitating as they went. A follow up eachelon could destroy the severed heads (immobilised but still active as long as the brain is undamaged) in their wake at leisure.
I had a friend once, but the wheels fell off. Sad, very sad.