Over time of watching the Walking Dead, I noticed that main characters adapted well to predicting walker locations and positions. These skills have definitely saved their lives many times. However, with time passing by we start seeing tons of crawling walkers that are hard to spot sometimes. Considering one bite is all it takes, why not shield their legs?
Any form of leg shielding would presumably slow them down. While walkers are slow, the ability to move through a "herd" is often necessary, and anything that impedes their movement would be more of a detriment than an asset.