It’s more so like reinventing the wheel. Certain applications of a CMS make sense to store data in flat files (blogs, ect) but for the bulk of use-cases flat file is either not advanced enough, or would be either practically impossible or practically too slow to really get any kind of benefit.
The same logic applies to using SQLite in production vs an actual database. SQLite has some great use-cases, they are just few and far between.