Well there are many reasons,
1. I am broke!
2. I ain't got a good job!
3. The diamond industry is a corrupt evil fucking mess of a organization. Profiting off of child labor, war, and abuse of monopolies. They control the supply of a rock that isn't even that god damn rare but don't ship enough to meet demand ever thus driving the price up. You are better off buying a glass replica for 20 bucks rather than a 5000 dollar chunk of shiny carbon.
4. Fuck You, They are shiny rocks, they have no real value outside of construction!

because they're overpriced chunks of organised carbon. a 50-lb bag of charcoal is $35 but a diamond smaller than chickpea can be $5000? bullshit, not worth it….