Several "Graph on DuckDB" efforts started on DuckDB and ended up reinventing a basic columnar codebase to innovate on because of these reasons. Even though people didn't document why, lack of outside contributor friendly flows is likely one of the reasons.

KuzuDB folks worked on something called GRainDB in 2022: https://vldb.org/cidrdb/2022/graindb-a-relational-core-graph...

But circa 2023 decided to write their own. Work continues as LadybugDB. One of my long term goals is to find an integration point with DuckDB's table implementation as the "node table". Conversely at some point DuckDB could implement all the join algorithms and LadybugDB's REL table in their code base.

For now, any talk of Graph on DuckDB is limited to DuckPGQ and the more recent entrant DuckGQL (both of which don't touch the storage layer which is the main reason why LadybugDB exists).