Also surprised it hasn't been feudalised somehow. Some company owning all the machines and then uber-ing out the hard work of siting them and restocking them. Gamifying the process of monitoring, "your fleet", the "ching" on your phone when someone buys a bottle of water or candy bar. Special deals for site owners. Branded supplies. An app for customers to talk to the vending machine. Loyalty points. A snack radar. Etc.
I think it's great the market is mostly independent operators.
Also big companies have too much to lose to fraud. The route operators are alone with big piles of cash. It is very easy for someone who knows what they are doing to "discover" a broken a machine (read they break it) and it was dispensing the J row without payment, then take the extra cash.
All in all it is best to leave route operations to little players.
The fact the big players don't bother, the number of 'entrepreneurs' doing this in their spare time, the fact that they repair, move and restock the machines themselves, the calculations which ignore the presumably high risk of vandalism or theft, the presence of a landlord who can charge you as much as they like. And finally, the existence of 'mentors' who will charge you to be taught how to replicate their business.
Edit: See https://en.wikipedia.org/wiki/Zero-profit_condition