Computers not connected to the internet can be hacked. It just makes it harder for them to be hacked. Some actually do isolate their internal network so that it isn’t connected to the internet. Others use firewalls and other measures to protect themselves. Even others have certain sensitive networks not connected but other systems are connected. Many connect to the internet since it may not be practical to isolate the internal network. For example, some employees may need access to the internet to do their jobs. The company may also have a web site and sites where you can download updates to their software.