Insecure dev machines.

Yes, this is kind of the key part here. If we cannot trust the developer workstation, we cannot trust any actions performed on that workstation, so sigstore's short-lived certificates don't solve this problem. They solve *other* problems, such as having a trusted public ledger and getting a proof that, at the time of the signature, the person still had credentials for that particular account and that account was active. However, if the workstation of the developer issuing sigstore certificates cannot be trusted, the signature cannot be trusted either.