Conceptual reality, mettā, ignorance

According to some commentaries/abhidhamma, all unwholesome states are rooted in ignorance; that ignorance is, according to Pa Auk Sayadaw, the ignorance that sees things as concepts, e.g., a man a woman etc. However, it seems like in order to do mettā and similar practices, one must take a concept as object. My question: Since wholesome states like mettā are said to be incompatible with unwholesome states like ignorance, why does it seem like mettā is rooted in ignorance?