Columnar DBS are mostly used for analytical workloads, where you normally have wide tables (analytical data with lots of attributes) and want to query only some

Columnar DBS are mostly used for analytical workloads, where you normally have wide tables (analytical data with lots of attributes) and want to query only some fields. You end up scanning most of the data, but because it is store in columns you only fetch the cols you need. In a regular DB you would fetch the complete wide row and then discard the unused cols. Regular(oltp) databases are probably optimized for certain use cases, and changing a storage layout midway may not be feasible if the DB has to keep supporting lots of online txs. I'm no DB expert but that's my educated guess.