The quartz watches are (way) better at time keeping.
And the rest is just a matter of paying for luxury.
The main reason the analogy doesn't really help is that the mechanical watch market absolutely cratered, and it's only being propped up by enthusiasts of cool machines.
Diamonds can't really have that. They've already scammed the general population into using slave diamonds at monopoly prices for 30 years longer than they needed to, but no one is _passionate_ about slave diamonds.
It's a poor analogy because mechanical watches and quartz watches aren't perfect substitutes and aren't purchased for the same reasons. Everybody's phone keeps better time than a watch; absolute accuracy isn't the exclusive measure.
Lab diamonds and mined diamonds are literally identical products.