All the carbon and oxygen atoms are sp2 hybridised and can have a p orbital in the correct plane yet only conjugation of the COO- group occurs. I can't think why. I can't draw resonance structures for the negative charge being delocalised around the ring but if you look at it from the orbital perspective (sp2 hybridisation) it seems clear that this conjugation should occur.
Here are some of the resonance structures we can draw for the carboxylate anion of benzoic acid.
You're correct that we can't delocalize the negative charge from the oxygen into the benzene ring. Even though there is continuous p-orbital overlap between the oxygen and benzene carbons, there is just no way to push the electrons so that a negative charge winds up in the benzene ring. However, as the resonance structures show, we can move electrons the other way and place a second negative charge on the oxygens and leave a positive charge delocalized around the ortho and para positions in the benzene ring. This is why the carboxylate anion of benzoic acid will not react with electrophiles (electrophilic aromatic substitution) at those positions in the aromatic ring.