PostgreSQL is more complex to use and operate and requires more setup than SQLite. If you don’t need the capabilities of PostgreSQL then you can avoid paying the setup and maintenance costs by using the simpler SQLite.
I have never had a Postgres install go that easily. There’s still initialization and setup of the server and users. And you’ll have to do something about upgrades as well. Postgres isn’t difficult to set up but SQLite is just a file. It’s much simpler.
And that’s only the installation. Interaction with SQLite as a database is also simpler.
They both have uses but it’s strange to me to assert that they’re equally complex.