> run on postgresql with no overhead at all

Citation needed. I love postgres as much as the next person, but it does have more overhead than Sqlite which is in an in-process db linked through compiled C code, it doesn't run as a separate server. Very significant difference that when you use Sqlite db, there is typically no db process other than your application logic, unless you implement the server yourself. If you don't want your application to have multiple processes (say, as a toy example) then it totally makes sense to prefer Sqlite just for this reason. Sqlite and postgres are different tools, they serve different purposes.