As an addition to the other responses about the historical side you can currently still ship your car in/out of Australia today you'll just have to do it as cargo instead of a single passenger ferry which also carries your car too. I know a guy that did that with his car to Europe for ~8 months, just make sure you plan well in advanced or you'll be waiting 2-3 months on that leg :).
Yes, passenger ships and ferries were more common than today. When my mum went to Australia for school in the 1950s she went by ship.
This sounds like a "back in my day I had to take a ship to school" bit
Ships were obviously much more common given there’s basically one trans-Atlantic ocean liner these days.
Yeah a lot of it was killed by cheap flights.
And even before flights got generally cheaper, it's hard for most people to justify taking almost a week longer, between a very limited number of destinations, on a pretty limited schedule.
Even in 60s when big himalayan expeditions were done, people travelled by big boats starting usually in India, for a month or two. Then got half of their equipment lost (so this part didn't change that much).
There used to be far more (sometimes ocean-going) RORO ferries than there are now, Herald of Free Enterprise and Estonia really reduced the market to areas where the ferries are pretty much essential.