I’d prefer not to be the first person running up against a limit or discovering a bug in my DB software.
Flat files have also been reliably used in production for decades. That doesn't mean they're ideal...although amusingly enough s3 and its equivalent of flat files is what we've migrated to as a data store.
Many people consider this most expensive bug in history, when on first flight of Ariane 5, it enters speed range, which was hard prohibited in Ariane 4 software and caused software exception and then 1 billion crashed.
Honesty, they could re-check all ranges, but they decided, it would cost like write new software, so to save money, was made decision to just use old software, without additional checks.