The reason why most people disagree is because,
1. It's not that easy to code (especially the fact that cc devs is well, y'know..).
2. It's gonna give you an unfair advantages because your friend could just join you and just go through walls and probably give you some information that would be unfair such as leaking enemy's position, either by coordinates or by telling which island they're currently in. You may just say "Why not just make it so players who joined you to spectate only see your perspective?". Well, that'll make cc dev's work harder and obviously they probably would not like it.
3. The ability to join your friend would be a bad idea since it'll basically silently promote cross teaming in solo games and would probably increase chances of teamers spawning in solo match and obviously CubeCraft team disliked that. You may be saying "oh well, just disable the ability to join others in solo match". Then that'll ruin the whole point of this suggestion as you could just use /party