ssh -A doesn't properly enable forwarding of authentication agent connection
Actually, the very simple answer is that you have to run

ssh-add


and then it all works. 

Basically, macOS already has the ssh-agent set up for you, but after each reboot you need to add your keys to it. ssh-add gives ssh-agent access to your keys for the current boot cycle.  You'll have to enter the password for your private key if you have created one.