Perhaps a lot of the open-only players are involved in the Thargoid War - a lot of joint activity in CZs and station defence going on.
I know nothing about Eve - but here you have some obstacles to overcome - 1) the sheer number of systems, 2) time-zones, 3) Odyssey and Horizons are separate (plus the legacy and consoles split) and 4) the aversion of people to any chance of non-consensual PvP encounters.
Since there are some who get their "fun" from building hugely powerful vessels and just destroying anyone they come across then there is a perception that this can happen just about anywhere so the temptation is to play in Solo to avoid it. Of course in truth, there is vary little chance of coming across one of these at random as they (like you did) sniff around where there is a good chance of encountering others. So, Engineer's systems, CG systems, Shinrarta Dezhra and places people use to grind Fed or Imp rank tend to attract them - (thus surprised you didn't find much at Cubeo).
If what you want is actual PvP then there is a system (San Tu) that they use for that, have a search for groups that organise that.
I used to play exclusively in a PvE Private Group called Mobius - always plenty of hollow icons around the place but for some reason it seemed to become empty, hardly ever see one any more. So now I tend to favour Open just for the adrenalin of seeing a hollow rectangle appear. Still, like you, I don't see many at all - but then again I don't do Open Play at Engineers or CGs etc. (I have no interest in combat.)
