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What about using plastic or some flexibly epoxy. Would it have some adverse effect or is there a cost issue?

Epoxy-coated steel rebar has the problem I mentioned, as well as being more expensive than standard rebar. You can remove the steel and use non-corroding materials, but these tend to have significantly less stiffness than steel (and reinforcing stiffness determines the load-to-crack-size ratio)

That is really interesting. I didn't realize that corrosive oxidation would become concentrated and react at a higher rate if the point of exposure was limited.

Why isn't it galvanized like a nail or a porsche?

It can be, but it adds cost to the project, and any galvanized rebar has to be bent before it gets galvanized rather than on-site.