Here's what likely happened -- they built the entire system from the ground-up based on a single-user because integrating with iCloud family wasn't part of the MVP.
By the time someone got around to asking about it, they realized that their data model was so thoroughly fucked it would take several weeks of risky migrations in order to add multi-user support.
So stakeholders are punting on it left and right. The things are selling, so is multi-user support really that important? It's an "edge case".