Re: Why not?
1) Expense: Why should a developer have to pay Google when Google will be providing no actual services to the developer? These are apps distributed and managed outside the Play store.
2) Vulnerability: Some two piece chicken mcnobody launches a spurious claim against the developer about one of their apps. The developer license is suspended for investigation, if not permanently. This kills every other app by the developer immediately.
2b) This affects legitimate developers who might have been breached to publish something under their license, or push a bad update.
3) Pointless: Google claims it will stop proliferation of bad apps. Outside of the play store, not only is it the business of the app store operator to manage their offerings, but often one must actively and deliberately seek said apps. If they do so unwittingly, they are otherwise so susceptible to "go here and install this" schemes that trying to secure them is futility defined, they will always find a way to stick the fork into an electrical socket.
3b) Bad actors will probably pony up $25 without hesitating because they stand to greatly profit. So the socket remains available for being forked, and can be accidentally forked by people who think being Google Verified is an automatic sign of trustworthiness.