I did wonder about that... but I think there were two main reasons;
a) That hard "rocker box" plastic would likely fracture, if it hit a speedhump something.. a metal one will just dent.
b) Metal is cheaper.
The one I bought was a Febi item, which (compared to the old/abused Fiat one) seems to be made from a bit thicker metal (just my impression).
I think the more robust metal is probably a good thing, since the rim will be more "precise" in manufacture and won't become deformed, twisted etc. in storage or transport, so will fit better (less likely to leak).
In any case, I rubbed the surface of mine up with a scotch pad (black/fine for all you detailing purists
) and gave it a couple of coats of Hammerite smooth (aerosol). Still looks new..
Ralf S.