Why is an Anti-virus able to scan the encrypted disk?

It may sound silly but how is an anti-virus able to scan the files on my computer(without me having to type in the encryption key) since all my drives are encrypted ? That doesnt make sense to me.

If you mean scanning them when the OS is loaded and the encrypted drives are mounted and decrypted too then it’s the same as with unencrypted drives. Your OS and antivirus see them as unencrypted.
The other thing is when you want to launch an antivirus scan when encrypted drives are not mounted and not decrypted. In this case an antivirus doesn’t see or understand your drives.

In other words, when you’re able to access you files, so is the antivirus and vice versa.