First, because it’s not a migration in the Bitcoin core sense, or the Signal sense, etc. In nostr the vulnerable key is the absolute end of the line, last station on the subway. Second because a hard fork requires consensus and there is no way to achieve consensus here on something like “just” swapping out the keys, that would require consensus on everything about the hard fork from everyone who is to be a lead participant in it, and in a highly organic and unstructured way, which is the only way nostr has. Which, if you think about it, means starting again from scratch.