Because such a set would lack playability and have zero mass appeal (you would never buy this set stand alone). There was an Idea submission for such a set and it was rejected bu Lego:
https://ideas.lego.com/projects/a88109ec-9970-4fe1-98b4-9bd5...
Additionally IMO they're missing the boat by sticking to their classic distribution model that has the restraints you say. I mean sure, some sets aren't fit for shelf space in a Target toy aisle, but they should be able to JIT packaging for just about any set that's had the work put in as long as they don't run afoul of trademark.
there won't be a trademark issue for the brick producer, as they are only delivering an assortment of bricks. the model design comes from elsewhere.
trademark may be enforced against uploaders of protected designs, but that is no problem as the design can still be shared elsewhere and it can't outright prevent anyone from ordering a certain assortment of bricks.