my wild guess would be they do not want the disparity on number of party characters between single player and multiplayer. if single player proven much more enjoyable than said multiplayer with 6 party characters, they would be slapping themselves in the face. restricting it to 4 makes the game more accessible to general audience. having more party characters would be a nightmare for them especially if larian's goal is to market the game to multi-platform be it on consoles and handheld. it certainly won't be an enjoyable experience for casuals especially if they have to control too many party characters.
that is the sole reason why they want it to gimped the game and forced it as a party of 4 (which you have exposed them of gimping the stats). best hope really would be mods but i really hope larian will really do the game justice and allow 6 party characters. looks how successful with pathfinder wrath of the righteous. they should learn from owlcat honestly.
But what doesn't make sense is if they are gearing it for multiplayer then they are truly shooting themselves in the foot with 4. You literally miss a LOT of origin character interaction with 4 player multiplayer because you CAN'T have a single origin character in your party. At least if they allowed 6, you could have 2 origin characters in the party and 4 players would still get the experience of all the origin character stories. As it is currently, you are severely restricted in 4 player multiplayer.
And it really makes no sense. Four characters should never equal a maximum full party. That makes no logical sense other than that this is a video game and the designers don't want anything more than a party of four. The only time four would max out a party is if the entire mission was some sort of super secretive stealth mission. Most of the Encounters in this game are not stealth missions.