That’s the point, you see? It’s about focusing on what we’re doing. Which is why it is recommended that we chant aloud, albeit softly so as to not disturb anyone else- it is all too easy to lose track if you’re chanting mentally. However, as we spend time in the association of God in the form of His Name, we will gradually begin to appreciate His presence and find it easier to concentrate. At this point, we will want to continuously be in His presence, and so start to naturally continue a mental chanting even while apparently engaged in day-to-day activities! That is the eventual goal for all of us. As an example, there is a mention in the Mahabharata about Arjuna chanting Krishna’s Names even while asleep. Also Haridas Thakur, who is considered the patron saint of the process of chanting (he used to chant three hundred thousand Names every day) used to chant one third of his daily quota in his mind.