Keep in mind as well that people who get attacked by lurkers doesn't meant walkers are smart. Instead, it just means people weren't paying attention (e.g. Lee distracted by looking for Clem).
At the end of the day, anything that has essentially no self-survival instinct are pretty dumb. Most animals in the wild are cautious/defensive against humans because they at least have the instinct to know we may be a threat. Walkers, you can keep shooting them one by one and they will just keep walking into your gunfire.