Home wireless networks can co-exist for a couple reasons. For example, I'll use 802.11g since it is the most common home wireless signal right now. It can run on 1 of 11 frequency ranges (channels) here in the US. They do overlap some, so you end up with only 3 that are useful in most environments. In addition to that is a network name, also know as a SSID that allows devices to determine which network is which.
For a common combine network, all devices would have to be on the same channel and SSID for the purpose of having a standard. Encryption in this scenario would be worthless. Whatever the method of encryption is, would be known by all devices used to talk on that network. This would have to be public knowledge, and therefore not worth the time to encrypt it in the first place. It would just add more complexity to the situation.
As for why it wouldn't work on a combine, well the whole machine is made from mostly steel. Radio signals in general don't pass through steel very well. Tin file cabinets cause problems in an office environment, let alone the many layers of thicker steel used between the cab and engine in a combine.
On top of that, it's not like it is very difficult to get a wire pair from one end of the combine to the other. This CAN be done with as little as 2 conductors. The wire looms that run through combines now isn't necessary with the correct components.