Everyone knows about it and yet everyone does it.
It is like getting a luxury brand car or a watch or living in a certain part of town. All aren't practical.
When social pressure is already there, you are asking the female partner to break the norm which can be tough.
Come on, it's 2016, the female partner can take care of herself. Valiant knights are not needed anymore, she's slaying dragons too.
It's even worse than those. A luxury car will probably be high quality, and might have extra features not standard on more reasonably priced cars. Living in a certain part of town is probably safer. A fancy diamond is only expensive because people think it can be expensive.
It shows how the concept of diamond engagement rings were completely invented by a specific ad campaign; first in America and now expanding to the rest of the world.
They were specifically trying to create social pressure to buy the most expensive ring possible; to reinforce the impression that a diamond ring demonstrated wealth and ability to take care of a spouse; and even set the three months salary purchase price target most people reference.
If there's ever been a greater con so openly and deliberately imposed on society, I can't think of one off the top my head.