Because the keys, from my understanding, change every day. So if someone is infected, you can only track them for the relevant days because those are the keys they would give you.
You could simply set it up to delete your own keys after 14 days, for example.
This is obviously just all obfuscation. The entire plan is that you are uniquely identifiable, you are being pinged by every person you walk near, there is protection from Target, Bestbuy, “Ze Russians” but not Apple, Google, Gov.
You can put as many crypto or “random” or key derivation layers you want in there, you just also need to admit they are surface deep and really “just for show”.
This is constant person tracking, that’s the whole point.
You could simply set it up to delete your own keys after 14 days, for example.