Why does the author put it this way? He's driving home the point that the normal song of the human heart isn't the song of faith. Although it works out in different ways, Christians and non-Christians share a similar struggle. We both struggle with unbelief.
Unbelief is serious - the writer of Hebrews has already labeled it toxic when he warned us against "an evil heart of unbelief" (Heb. 3:12, KJV). So we shouldn't get to Hebrews 11 and find a lot of sympathy for unbelief.
The writer there tells us that without faith it's "impossible to please" God. Not tricky, not difficult. No, it's impossible!
I don't think we tend to feel as strongly about unbelief as God does.
Unbelief is a decided distrust in the promises and character of God. Spurgeon describes unbelief as a "mistrust of the promises and faithfulness of God."
- Dave Harvey, from his book, "Rescuing Ambition"