When the game spawns the launcher tells it how many players will be joining. The game then spawns the universe based on that player count and creates the homeworld systems which are fixed in number. If more players were allowed to join after the start, there would not be any place to put them.

Any change to the launching system requires changes to the proxy servers, the launcher and the game servers. It's not trivial to modify and I'm just not touching it for now.