I've been reading lately about the Double Ratchet protocol, how it works conceptually. I understand generally the idea behind the protocol, that it provides Forward Secrecy and Post-Compromise security (the healing period depending on how often the shared secret is updated). The shared secret is updated by using the DH key exchange.
The thing that I struggle to understand what are benefits of this protocol over just the ephemeral DH key exchange, i.e. each time 2 participants generate new pair of assymetric keys, share the public parts and obtain a shared secret by applying DH to it? After that they may use some KDF to obtain a symmetric key to use for encrypting the message. To me, it looks like this provides the same properties (Perfect Forward Secrecy, Post-Compromise Security).
Why is the Ratchet Protocol is considered a industry standard for pairwise E2EE communication then? Why do we need the ratchets?