Think of it like this: what's the surest, simplest, and most reliable way of proving to people that you're wealthy? It's taking a good chunk of your cash - like, in the order of monthly living expenses of people around you - and setting it on fire. Or spending it on something just as useless - like a diamond ring, or that romantic trip across Europe.

That's why jewelry with obvious resale value doesn't work as well, nor does anything that can be seen as investment or an utility purchase: the signal only works if you're wasting money on a vanity purchase - one large enough to show that you either have more money than you know what to do with, or at least that you care so much about recipient that you're willing to sacrifice a meaningful chunk of your wealth on a gesture.

Of course few will say this out loud, or even think about this explicitly. Diamonds, engagement rings, and all the other staples of romance are in big part a status game that people learn to play from childhood, so to most, those things feel like something natural to do.