They already said in the livestream that a percentage of the DOS2 engine still remains in BG3, and that percentage is decreasing each day, presumably this includes assets. They are initially using old assets as EA placeholders so they can focus on the development of new systems, characters, quests, etc which need more EA testing time. This avoids the developer from having to focus on reinventing the wheel up front and later rush out an unpolished mechanic. Using older assets as place holders is considered standard practice in game studios.
