Well, the internet started as fragmented networks. They call it inter-net because there are existing local nets. Exactly what the inter-net does is not to absorb all the local nets into one huge net, but rather figure out a protocol of packet switching. Which means in concrete terms, if a network has its internal language, it figures out a way to send this language through some kind of translation called a protocol, so the other network can also understand that. The network operator doesn't need to do it. It's only one person here and one person there that has the encoding and decoding capability. This is called the end-to-end principle.