Why wouldn't a malware author just... do that?
Also so the os doesn't have to repeatedly rescan apps, etc.
All major Linux distros for example still have no viable way of creating signed programs or anything like Gatekeeper.
When a piece of (signed) malware becomes even moderately sucesful, Apple shuts down all its installation vectors by banning every dev account that's ever signed it. If nothing else this makes it harder to develop malware that spreads rapidly through the App store, forcing bad actors to invest a lot more work into finding 0-days.