This is the way I understand it and I am no expert on it, so someone else might have a better response.
But packets that are transmitted on a wireless network that is protected are encrypted as they pass "through the air". They are decrypted by the wireless access point and then transmitted via ethernet from there.
Ettercap is an ARP spoofing utility. ARP is an ethernet protocol. So the only way you are going to get Ettercap to work is if you are first authenticated with the wireless access point so that you can talk ethernet to the victim.
You can't use ettercap to sniff packets "through the air" as those packets are not using the ethernet protocol that ettercap understands.
A wireless NIC that is in monitor mode will pick up wireless packets as they are transmitted "through the air." This means that you are capturing the encrypted form of the packet. To break through the encapsulation and read the packet data payload, you'd first need to decrypt each and every packet. But you won't be able to do that "on the fly" to create a MITM attack.
If you didn't want to connect to someone's router before ARP spoofing, then you could also create a fake rogue AP. There is a video HowTo on these forums: