I see most of them are made out of stainless, why not hard anodized aluminum? i thought that was pretty resistant to corrosive stuff

Stainless is the only way IMO.

But why is that your opinion? Do you have any experience with hard anodized aluminum?

IMO eventually the anodized coating will start to compromise and the aluminum will corrode espically in salt water. Good quaility stainless like 316 will last basically forever if taken care of.

Doing a lot of my boating in salt water if I was going to buy a boat I see the aluminum pieces as a negative. I Also see painted trailers as a negative.

I just recently bought a Hardin Marine 316 stainless strainer. It was a little more expensive than the 304 stainless version. When I was looking to buy the strainer I stayed away for any that had any aluminum.

Just my. 02

I see this corrosion all the time from Lake Norman in NC , it looks twice as bad as Salt Water corrosion.
I always wondered if it had something to do with the nuke plant!

For boats that are regularly used in fresh water lakes like this, there is a different type of anode that should be used. IIRC, it's Magnesium.
Mercruiser Sterndrive Magnesium Anodes

Strainers are expensive to begin with, why cheap out to save a few bucks, or use an inferior material. Go with the Stainless 316 and call it a day. Proven, reliable, used for years.

You might try to build one with duct tape. I heard that works incredibly well :-)