To replace a worn out drum only is why.
If you'll replace the hub you'll need to keep the bearings as matched inner and outer parts. To buy new bearings and grease seal would be optimal.
The better question is why the drum was swedged to the hub. It's much like the left hand threads on left side of the car. Or the odd little clip on a contact stud to ventilate the mechanical limiter mounted inside a fuel gauge. Yeah that went away too. All examples of what their engineers deemed necessary only for a period of time.