That was and still is the case. We just have forgotten about it. In the past it was SETI/napster/torrents/kazaa etc. Someone else mentioned minecraft servers. There are also lots of people running similar opensimulator servers at their home. Unfortunately laptops/tablets have thinned that crowd and the possibility of having such arrangements is smaller overall now.

I presume you mean "never the case for many people". As in "most people never left their PC on 24/7". That may well be true.

I used to run a couple of minecraft servers for my kids. Judging by how amazing their friends thought that was... I don't think there were many other people in the town doing that.

On the other hand, I still have a raspberry pi running 24/7 to this day.

Of course it was the case. What do you think things like Seti@Home were running on? People who owned computers had so many idle processor cycles that they were desperate to find something to do with them.

If 24/7 home computers were never a thing, what did BBSes run on?