“Why are they machining this out of a solid block of aluminum, instead of investing in foundary tooling so they can cast these ?”
That is a very good question... As a fabricator I would guess that it might be due to the fact Forged Billet Aluminum is much stronger and less prone to fatigue. This is why they stopped selling cast aluminum semi truck wheels many years ago, they are now all forged billet and machined. I can just imagine the forces placed on it in this application.
Machining these from billet only needs one shop, one machine, and one or two setups. With casting you need a foundry, wax molds, and you still need the fancy cnc for the finish pass to get it balanced. Then ad the QC process transfering between shops and processes, billet ends up being cheaper.
Forgings can be stronger, but it also depends on the alloy, heat treatment, stress relief etc.
What I read here, Mercury uses a proprietary alloy and all their propellers are cast.
https://www.mercalloy.com/mercury-castings/
I don’t know about all the variations in propellor pitch, but this one look like a good candidate for lost foam, although I guess it depends on volumes.