many other industries use injection casting of aluminum alloys to make parts which must withstand MUCH greater stresses than a fly reel ever will. Firearms and aircraft are two which come instantly to mind. USUALLY but not always, these parts are cast somewhat larger than the finished part and then machined to the final dimensions.
I wouldn't think any company is going to take the time and effort to produce a fully machined reel which is a piece of junk.
A reel made with cast parts can be every bit as "well made", "strong", "good" or whatever adjective one chooses, as a fully machined reel.
Conversely, low-quality, low-tolerance castings can be made and sold by the boatload for cheap.
We're talking about fly reels-- you're not going to drive nails with it or use it as a car jack, or a lawnmower. I've never had trouble with cast aluminum reels, in salt or fresh water.