Heavy modern metal cases of anodized aluminium, extremely polished steel or brass etc. for keyboards are the equivalent of driving a gold plated Lamborghini on city streets - bordering on pointless vanity. Not only is metal as a case material completely unnecessary (I've never seen even a 10$ rubber dome with a plastic case significantly wear under normal use) it adds nothing particularly tangible to the durability of the keyboard itself since the case just hides the same, more or less cheap PCB's inside that every mech keeb has. At least an expensive sports car gets you a sports engine. Metal cases serve nothing more than for conspicuous consumption and to create artificial luxury tiers in the hobby.
Metal cases aren't as expensive as you make them out to be. You can, like me, buy a well-made full aluminum case with an insert for somewhere around $100 from KBDfans to try out a metal case keyboard, and it noticeably alters the typing experience. Not saying that it's superior, but it is very obviously different and considering that everyone here had been typing on plastic-case MX keyboards for years to decades, it's no wonder that a more expensive new alternative is prized over default plastic. Because let's be real here, what would differentiate, say, a custom keyboard with a plastic case from any stock board with the same layout? There's only so much you can change.
I personally still use and prefer a plastic case Realforce over my metal-case MX keyboard, as it's comfortable to use; many like the even softer experience of an HHKB, which lacks a stiff plate.
Anyways, that refutes the point of it being completely unnecessary, as it was never about durability, it was always about typing feel and a different experience.
Onto pointless vanity, that much is most likely true! However, many people in the community make a good living. To them, a thousand dollars or more on a keyboard is an insignificant amount of money, so why not bother making it as nice as possible to show off? People like having nice things, and keyboards are not an expensive hobby by any comparison.