In theory, you don't. In practice, it's because the major SQL DBMS were architected around row-oriented storage and the technical effort to implement hybrid storage is large.

There are columnar storage engine extensions for many of the popular databases, though.

there are hybrid engines too, so-called HTAP (as opposed to OLAP or OLTP) notable efforts are: SingleStore (commercial) and OceanBase (foss)

Interesting: I transferred the idea of a matrix being stored row or column wise to the database world and assumed this was a more physical than theoretical feature (not a native speaker here)?

Looking forward to check out `pg_duckdb`, yes.