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 wonâ€™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."