They're living in homes that are worth a lot, but are not willing to leverage the value towards more flexibility in their lives. Other people we know own property free and clear, but still don’t see themselves as adequately protected. They don’t want to take the risk of letting go of their work, or even reducing their hours. A huge part of retirement is about trusting yourself and your ability to adapt to a changing world.
That quote (especially the final sentence there) really stuck out to me as an aspect of mustachianism that a lot of people I know who are responsible in terms of accumulating wealth through good choices can't seem to wrap their heads around. My parents, for example, could retire early, but the thought never crosses their mind since they enjoy what they do and have become good friends with who they work with. Even if they were convinced that they never had to work again, they would probably just scale back their hours.