You can make more by having more machines. How many machines could one person restock? A lot more than 13. Let's say I have to visit each machine once a week on average (seems reasonable given it's sold $75 worth of stuff), I could stock 100 machines spread out over a small city. A lot more if I have many machines located together in say a university or an office park.

The average operator has 13 machines and views it as a side hustle. But that's true for a lot of entrepeneurial businesses. (The average SaaS owner, the average Youtube personality...). That doesn't mean you can't scale the business to where it provides a very decent income stream.