As to why you can't do it while in Solo mode, I'm pretty sure all three modes (Open/Private Group/Solo) use different servers in the server cluster. Also, they probably have different networking rules. However, this is just speculation on my part based on observation of how the modes work, etc.
I am pretty sure that SOLO Mode is simply the same mode as OPEN with Matchmaking turned off. Depending on your latency with Frontier, you can often find yourself in an instance in OPEN where you are consistently the only human player in the game for hours on end. Even at popular locations.
For some reason, I have a consistently high latency/ping with Frontier (500ms avg) and as such, I often experience this. No other game server I play on has anything higher than 80ms, with most being in the 30-40ms range, but I digress.
So... Solo with MM turned off would more or less give you that same single player experience.
GTA 5 Online players can achieve this same OPEN but really SOLO effect by setting their MTU value MUCH lower than the acceptable value. The game will see that and put you into an Open session, but with no other human players ever. A way around the new features in GTA 5 Online that "force" players into an open session to use all the options.
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