Because Postgres is another thing you have to run, maintain and monitor, and you have to make a judgement whether that extra complexity is worth it - for a lot

Because Postgres is another thing you have to run, maintain and monitor, and you have to make a judgement whether that extra complexity is worth it - for a lot of simple projects, it's not. SQLite is a library you use in your application process that writes to a file. There's a bit of care you have to do to ensure you back up that file safely, but there's no extra monitoring or maintenance above what you do for your app anyway. I agree that running Postgres isn't terribly difficult, but no matter how simple you try to make a small Postgres instance, SQLite is simpler.