That's just it though, there are people in the Mobius group who only play in Solo, they simply joined the group to have people to chat with while they play in Solo. There are people in there who play in Open except when certain friends in the group are on, then they play with them for a bit and go right back to Open.
It's not a large chunk of people ALL playing in the group setting, it's just a lot of people who joined a group.
You really don't get the scale of this game do you? The bubble, that little itty bitty area of the Milky Way galaxy that Humanity occupies right now, there's over 100,000 systems in that area of space alone, 20,000 of those systems are inhabited BY Humanity. The galaxy itself, 400,000,000,000 systems in it. THAT is our game world, and it's all on a 1:1 scale, so an inch in the game is the exact same as an inch in reality. Hutton Orbital, did you know that that station is over 2,000,000,000,000 kilometers from the jump in point? 0.22 LY distance, 89 minutes of travel in supercruise, the farthest distance we have to travel in supercruise to reach a station in the game currently.
There are 1.4 million copies of the game sold as of December, not all of those are actually unique players, but even if they were, that's only 14 people per system if we ALL logged in at exactly the same time and we were all in the bubble and we were all in Open. Now, considering that at any given time there's hundreds of thousands of people outside of the bubble on exploration trips, that number drops a lot. Add in that we aren't logged in at the same time and that number drops even more, time zones and geographic locations, work schedules, free time...with luck, there's maybe 20,000 people logged in during the peak, maybe. That's 1 per inhabited system, that's it.
So, the reason you don't see lots of other players in Open has nothing to do with EVERYONE hiding in Solo, it has to do with simple basic math. The odds of you encountering another player at random in the game are, literally, astronomical, fitting isn't it?