Datasets are often stored in (sometimes gzipped) jsonlines format in my field (NLP). The file size could reach 100s of GBs.
In those cases, querying un-indexed files seems quite a thinko. Even if you can fit it all in RAM.
If you only scan that monstrous file sequentially, then you don't need either jq or jj or any other "powerful" tool. Just read/write it sequentially.
If you need to make complex scans and queries, I suspect a database is better suited.
Databases are not used in this case because it’s a complexity overhead compared to plain-text files. The ability to use unix pipelines and tools (such as grep) is a bonus.