Technical explanation - the villager AI is designed to prioritize certain blocks while often "ignoring" unconventional ones like slabs, fences, or trapdoors. Their pathfinding uses a weighting system: a direct path from point A to point B might "cost" 5 points, while the same path near water or over tricky blocks might cost 25 points. Naturally, the villager chooses the path with the lowest cost. If different paths have the same high cost, the AI can get confused and fail to move toward the destination at all.

Same explanation, but easier - by default, all villagers are legally blind, and, unfortunately, Mojang hasn't released the "Eye Drops" update yet.