I don’t want to give other teams direct access to a DB and have them Not only take a dependency on the schema, but have the ability to run arbitrary queries that may exhaust resources in ways that impact normal operations. If I expose an API, I control the access patterns and can evolve the schema separately to suit the workload.
If other teams need a replica to perform their arbitrary queries, I’d much rather have them using a richer data model that they can normalize into whatever form suits their needs than have to conflate that into a source of truth data store.
If you have a single business unit and can get away with commingling concerns within a small team, great, throw it all in a single DB. If it makes sense to split, however, do it quick and early to avoid a decoupling hell that is more expensive then having split in the first place.