Recover from compromised shares with Shamir Secret Sharing
The point is that the dealer generating the update needn't know what the shared secret is.

If we had a dealer that remembered what the shared secret was (or we asked enough people to contribute their shares so that the dealer could reconstruct it), then yes, the dealer could generate new shares.

However, this would require is a dealer that did know the secret. In contrast, anyone can generate a random polynomial with an constant term $0$; all they would need is the degree of the polynomial (that is, the threshold of the secret sharing scheme), the field the polynomial is in, and the $x$ coordinates of the uncompromised parties; all this can be made public without compromising the security of the system.

Remember, the whole point of the shared secret scheme was that no one knows what the secret is (until it's officially time to reconstruct it).