But most projects don’t even have customers when they start, let alone large quantities of their data and legal requirements for guaranteed availability.
I think it's reasonable for a business, even a freshly starting one, to expect to grow to the point where it does have enough customers to outgrow SQLite fairly soon. Between that and PG offering more features in general (which often simplifies app code, because you can do more with queries), it's still not clear why not start with PG in the first place.
PG, on the other hand, can scale enough to cover foreseeable future needs for most businesses, so aiming for something more complex than that is almost always premature.
But most projects don’t even have customers when they start, let alone large quantities of their data and legal requirements for guaranteed availability.