A lot of storage is a real estate play. Buy land in the direction of where cities are growing. Pay your mortgage with storage facilities sitting on top until the land appreciates.
The smart ones optimize towards revenue per square foot and can pull in upwards of $30,000+ of profit per month. And that can be on a single facility. Typically the smart ones own 2-3 facilities. They buy an underperforming facility and optimize its functions and reap the rewards.
How do they do it? One method (of a few) is by constantly pushing the rates up and churning customers that have lower rates. Once their occupancy hits a certain threshold (typically above 85%) they start really trying to push customers out with higher rates. If the occupancy starts to dip below their target, they start to offer deals. That way they can keep the monthly recurring revenue flowing while still optimizing towards revenue per square foot.
Edit: This is why there are specific REITs that focus on self-storage.