I think 80k IOPs on gp3 is a newer release so presumably AWS hasn't updated RDS from the old max of 64k. iirc it took a while before gp3 and io2 were even available for RDS after they were released as EBS options

Edit: Presumably it takes some time to do testing/optimizations to make sure their RDS config can achieve the same performance as EBS. Sometimes there are limitations with instance generations/types that also impact whether you can hit maximum advertised throughput