One of the big advantages people enjoy is the elimination of the network latency between the application server and the DB. With SQLite your DB is right there often directly attached over NVME. This improves all access latencies and even enables patterns like N+1 queries which would typically be considered anti-patterns in other DBs.
If you think they can't do this task well I encourage you to try feeding ChatGPT some long documents outside of its training cutoff and examining the results. I expect you'll be surprised!
I enjoyed this post, but I'm eager to hear what the next step would be for a real "production" userspace driver. Are these typically just daemons that are configured to run at start up? And then some configuration GUI communicates with it over a socket or something?