A lot of these aren’t defaults because of backwards compatibility. IMO there is no reason to not use WAL mode, but it’s not default because it came later
As long as all the processes are on the same machine the wal mode is a good idea.
It's not good in the case where multiple machines are sharing the same database. Like say if you had a shared settings file which allowed multiple VMs to be set in one place.
Obviously the same-machine situation is the most common. But you asked for a reason for when wal is not appropriate.
