Both anodizing and oxidation of aluminum look different from polished steel. Notably, oxidized aluminium loses its shine. Moreover, anodization can get scratched after which you get the ugly oxidized aluminium.

All of this is to say that as a "Symbol of lasting love" every-shiny platinum jewelry seems to make some sense.

Although in theory you could scratch anodized aluminum, the anodized layer is both harder and more abrasion resistant than the underlying raw aluminum. If you're thinking about scratching chrome-plated steel, let me ephasize that it is much harder to scratch a hard-anodized finish. The only anodized-aluminum object I have ever known to get scratched is the older iPhones (5?). That's probably a combination of people physically dropping them from heights and Apple's choice of a thin, relatively soft anodization process.

A clear anodization finish leads to "bright aluminum" - the natural shine is absolutely preserved. Natural oxidization on the other hand does dull the metal.

Yes, all forms of aluminum look different than steel. Gold and platinum do have the advantage of being homogenously non-oxidizing. But I think anodized aluminum is a great alternative with many advantageous properties for jewelry (low cost, wide variety of beautiful colors, high strength/weight ratio, low density, non-allergenic). I'm not sure why it's used so infrequently - perhaps because of the connection with aluminum cans and thus the image of "cheap"/"disposable"?

Platinum is significantly heavier than titanium, steel, and aluminum which adds to its perceived value.

"Anodized aluminum and titanium don't rust - and if by whatever means you DO allow them to oxidize, the oxygen-bound outer layer forms a protective barrier against further rusting."

So what? But they do melt. And you can't eat Titanium or Aluminum. We would starve to dead soon without Pt/Rh