A severe simplification of the history, but: In the 1960s, say, if you lived in a town with shit TV reception the local authorities might set up a really good TV antenna on a nearby hilltop and run a wire through town that everyone could connect their TVs to. This was called Community Antenna TV, or CATV, which later became known as Cable TV. The coaxial cable used for this doesn’t carry signalling like, say, twisted pair; instead, the purpose of coaxial is to provide an enclosed, shielded tunnel for radio signals to propagate along. The signal would fade over time, so repeaters would be added every so often to boost the signal and filter noise.

So, yes, all your neighbours can ‘see’ your data, because you’re all sharing the same coaxial cable, though it’s encrypted between your modem and the cable company’s local headend. Those boosters I mentioned would historically break the Cable network into neighbourhood-sized chunks preventing the modem signal propagating too far, so there would be a local headend within the same segment for your modem to connect to. The bandwidth available is split between all the users in the segment, so having a second coaxial cable coming through the wall would be of limited utility; it’d be easier for your ISP to just allocate more bandwidth to your existing modem.

You mentioned Ethernet, but in most Ethernet networks we use switches that ensure that only the recipient gets to see the packets. In the old days we used hubs, which are more analogous to neighbourhood cable networks in that regard.