IIUC, old cable modem networks were all one simple circuit, such that there was no unicast traffic. Much how Ethernet hubs used to work… everyone who transmits would be communicating with everyone else’s cable modems on the same node, not just the gateway. So it was trivially easy to spy on others’ traffic, and if you plugged your computer straight into the cable modem (and didn’t use a router of your own), it was pretty much as if you were on a LAN with everyone else on the same node (basically your whole neighborhood.)
In the beginning of cable modem rollout, consumer routers were not yet common either, so most people were plugging straight into their modem. Cable companies encouraged this, and would charge for additional cable modems if you wanted to use more than one computer.
You don’t need a hub; if all the computers are on one LAN they can see each other, that’s the whole idea. The difference with now is that back then you’d have one computer directly connected to the internet, while now you have a router connecting your home network with the internet using NAT.
What I mean is that back then, you were on the same network as everyone else on your _block_, not just your household. You couldn't send packets just to your ISP's gateway, due to the nature of how the coax cable was wired, you're sending packets to everyone else connected to the same ISP node. (Ethernet as a protocol is designed to ignore unicast packets that are not addressed to your MAC address, which is why this works just fine.)
You couldn't connect a simple hub with multiple compuers to your cable modem back then either, IIRC because your MAC address had to be registered to their gateway, and it didn't really tolerate having multiple MAC addresses on the same cable modem. Which is why consumer routers started to get popular, because it allowed you to "share" your cable modem connection to multiple machines in the same home.
In the beginning of cable modem rollout, consumer routers were not yet common either, so most people were plugging straight into their modem. Cable companies encouraged this, and would charge for additional cable modems if you wanted to use more than one computer.