Can't we just make IV as unique and secret? Will it mean that the IV
can now be treated as a secret Key?
Key distribution is hard, and there's no reason to make the IV a secret.
If Alice wants to send a message to Bob using symmetric encryption both must somehow already know the encryption key and the IV. To share the encryption key Alice and Bob could meet up in person once, then use the shared key to send many messages without having to physically meet again.
Now if the IV is a secret, and the IV must also change every message then Alice and Bob now have to meet to share the secret IV for every message. If they have to meet every time, then why not just share the message when they meet?
Seeing as the encryption is not weakened by an attacker knowing the IV, they might as well save themselves the hassle of meeting in person by just sending the IV with the encrypted message, rather than trying to securely exchange it. It saves them both a lot of hassle, and they're no less secure.