Platinum has very low reactivity so it doesn't rust, corrode, whatever easy, unlike the others. That's why it's considered valuable, because it looks almost the same after a lot of time.
Neither of those metals becomes structurally weaker in any meaningful way over time due to oxygen exposure.
So what? But they do melt. And you can't eat Titanium or Aluminum. We would starve to dead soon without Pt/Rh
All of this is to say that as a "Symbol of lasting love" every-shiny platinum jewelry seems to make some sense.
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"?