The problem is control of the network. If you don't control your network, then you don't know if network provider will go from "unlimited data" to "I am changing the deal now it is 1USD/GB" and you can either take it or be without connection which would likely collapse your operations. So the only logical step is to use network which you have full control over, like WiFi.

The much higher speed also does not hold together when you will look at it. When you have a device which is generating or processing a lot of data (GB/s ~ TB/s) such device needs substantial computational power to pull it off, so it is likely a stationary device connected to 230V. So why won't you drag a fiber there?