Not saying it's not a fair point however surface area and materials matter too. A bullet is a concentrated mass made out of a relatively strong material. A piece of wood even if it's sturdy is not the same.
If they were to enhance the protection I suppose they would go from something carbon based instead of heavy metals. Circuit cars have such "survival cells" which are subjected to several tests including intrusion tests. F1 helmets are bulletproof exactly to protect against intrusion. Obviously a rally car is different but I'm sure they could do better.
Maybe it's too expensive for WRC but better is always possible.