We could also do that, but it doesn't have to be the only solution.
Two problems:
1. it is not incentive-compatible (why would I retweet a thing I wouldn't have retweeted in the first place just because the person is new? This will bloat my feed with an opinion that isn't even mine etc).
2. it doesn't work if you don't have anyone following you at all. Like Shakespeare, imagine if he is revived and starts writing new plays on Nostr? He is really Shakespeare, but he doesn't know anyone, all his friends are dead.