How can you store program state on disk like that? You'd need serialization and deserialization. A DB might be easier.
You need serialization and deserialization when interacting with a DB, too. You're trusting a library to do most of that for you. This is the difference between a junior engineer and a senior engineer. As a junior engineer, you don't trust your own judgement and use a library. As a senior engineer, you don't trust the library writer's judgement, so you write it yourself.
What? No. As a senior engineer you realize that loading data from a database is a solved problem, has nothing to do with the core use cases you're addressing, and would be a massive waste of time to build yourself.
That would be really inefficient though. I vaguely recall database driven blog and forum scripts taking over because of how poorly the old solution worked at scale.