OLAP vs OLTP. Sure you could use one for the other, but they have ideal use cases. Updating a single record in SQLite is going to be more efficient than doing the same in DuckDB.
They seem similar at a glance but they’re quite different. You can think of SQLite as a transactional database while DuckDB is better used as an analytical database.
I can see applications having valid reasons to use both. You can use SQLite as the catalog in duck lake systems, for example. SQLite is your metadata record, DuckDB is your ingestion/scanning/aggregating/joining engine.