The point is there is no public key capability in BB84 that requires pre-sharing a symmetric key.
You absolutely do get forward secrecy with pre-shared keys. You just need to make the protocol derive the next key with a cryptographic hash function, and deliver the iteration count with the packet so the recipient knows which key is the correct one. This is called a SCMIP or hash ratchet, and it's used e.g. in Signal protocol.
(As implementation details, you'll also want to hash the hash ratchet counter with the key to prevent theoretical loops, and you'll probably want to encrypt the ratchet counter during delivery with static header key, or the very least authenticate it.)