psql does not require a password on macOS with Postgres.app
Found the answer in another post on stackexchange.

I've edited the authentication lines in pg_hba.conf and changed "trust" into "password" for all lines. It now looks like this:

# TYPE  DATABASE        USER            ADDRESS                 METHOD

local   all             all                                     password
host    all             all             127.0.0.1/32            password
host    all             all             ::1/128                 password
local   replication     all                                     password
host    replication     all             127.0.0.1/32            password
host    replication     all             ::1/128                 password


Now psql asks me for a password every time.