In all reality, we probably could have. But, wood isn’t as common on production knives as it once was. When considering that these are lightweight knives for hiking and backpacking, wood is even less common.
That said, wood makes for great handle scales on knives. You’re more likely to see wood on fixed-blade knives, but there’s a handful of folding knife models out there that still use wood. The Bushcraft crowd likes wood handle scales quite a bit.
P.S. We didn’t mention bone or leather handles, either. Or horn. Or copper. There are so many different materials out there. We stuck to the ones that pertained to the knives in this guide.