I don’t know. Historically, maybe for consistency across platforms? I’m fine with enabling it by default on Windows. I can create an issue for that on the tracker, unless you beat me to it.
Also, going back to my idea from earlier in this thread:
Now, that proposal is doomed (IMO), since someone is going to protest about the SQL API being denied. However, could expose the SQLite C API sqlite3_db_config in sqlite3 (together with SQLITE_DBCONFIG_ENABLE_LOAD_EXTENSION) so folks on macOS can enable extension loading the safe way. That would work on all platforms, so, as a side-effect, your app would be more portable.
Alternatively, we could tweak the sqlite3.Connection.enable_load_extension API to use sqlite3_db_config if sqlite3_enable_load_extension is not available.