a few reasons: 1) it could in theory work as an "extension" on top of something like Postgres, except such extensions aren't usually supported by most cloud providers 2) having it "built into" an existing DB only works so long as you're only using a single DB... but SpiceDB and Zanzibar were designed for multi-application permissions 3) SpiceDB's caching and other dispatch logic would not work if built into another system, removing some major benefits