Because an RPG like BG3 is not meant to be played solo. Neverwinter, yes. Skyrim, yes. The Baldur's Gate series is built on being a party based series of games, and that is how D&D is meant to be played as well, as a party. If you want a solo game, there are plenty out there. What you seem to be trying to do is force a game to be both solo and party based and somehow balanced for both when it is the 3rd game in a series that has been based on a 6 person party from the very beginning.
Well, it's developer who decides how their game is meant to be played and Larian confirmed that they will add a Lone Wolf mode.
"Is there a Lone Wolf mode planned? This is actually a question already being asked by a number of our playtesters who are currently playing the game. A Lone Wolf mode is planned, but won’t be in the game at the beginning of Early Access. One of the many great discussions that come from people playing, and talking with us."