I bet it has something to do with the licensing and content gate-keeping. If WotC was going to license world/campaign building, it would probably be a separate license and require more adherence to the game system. And, frankly, with D&D One coming out there would be no good reason for them to license an application based on the old system. As for Larian, they have more incentive to gate-keep content creation for DLC and the inevitable BG 4+. There just isn't much of a profit motive for either to want a capable DM mode out there.
This but even more. Hasbro/WotC are putting a huge amount of effort into creating a comprehensive online VTT/storefront full of microtransactions to fix their stated issue of D&D "currently being under monetized." A DM mode in BG3 would be a direct competitor to that, and so at very least WotC isn't going to give Larian permission to create one.
Our best hope is Swen's indication that BG3 will fully support modding. Ideally Larian will release better tools for that and the community will take up the creation of quest/adventure/etc modpacks.
