The modern-day aesthetic of diamonds is just that they are expensive. They're not distinguished by utility, quality, or appearance from cheaper products. The ultimate status symbol, but also obviously a bit of an issue...
(Moissanite is even better, so it should be preferred over diamonds unless I’m overlooking some other difference in their attributes?)
But plain glass gems look comparatively bland when used as jewelry.
Diamond (10) is 4x as hard as corundum (9) which is 2x as hard as topaz (8).
Glass doesn't sparkle.
https://www.walmart.com/ip/Miluxas-Women-s-Glitter-Tennis-Sn...
Diamonds (and other gems) really are beautifula to look at in ways that glass just isn't. And manmade ones, sparklier still out of the forge of our own cleverness, are much nicer in my opinion.