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.
"Average" (of 23 survey respondents) might not be a particularly great measure. I can imagine many operators would have 1-3 machines on the premises of the business they are actually focused on (a pizza shop or an office cafeteria with a drink and a snacks vending machine), while others might have 75 or more.
(How much does the average mobile app make? Does that low 4-figure amount (edit-to-add: per year) mean that app development is a dead end?)
Do you need to put aside money for insurance, ongoing repairs/maintenance, restocking physical goods, vandalism/loss of the machine for apps?
I think vending machines are not really a realiable income when looking at the costs that might occur.
So, I'd take any 4-figure app over any amount of vending machines.
>We surveyed 23 vending machine owners with various-sized operations and found that the average operator in our sample owned 13 machines that gross $309 per machine per month.
Warning: napkin math.
$309 x 13 x 45% = $4017 x 45% = $1807.65 profit (excluding cost of machine)
It's just a side hustle. Even with higher margins you can never go above $4017 so it's never going to turn into a scalable business. Once you consider the cost of the machines of around $2k each (very low estimate) your first 14 months will net you nothing.