Better machinability allows the production of finer cutting edges.
For heavy use some hardening and tempering often goes a long way.
Followed by a touch up on the cutting edge by very fine diamond paddles.
Most of my machining is in aluminum fixtures.
HSS can make a better surface then carbide ever could.
Carbide is a composite material of extremely hard steel held in a (commonly) nickel bond.
HSS granularity is much finer.