They value the aluminium keyboards only because they are limited. That is because they are collectors in the first place.
Why would aluminium be superior to plastics, seriously? Look at german cars, do they use a lot of aluminium? I don't think so. Maybe for the chassis, but not for the parts that are touched, for example the stirring wheel. Stirring wheels are always plastic. Even the ones that look like wooden wheels are plastic. The whole interior is either plastics, leather or artificial leather.
Its like with the red switches, when they were unobtainable, everyone was like OMG CHERRY RED SWITCHES, now they are (almost) publicly available and people don't like them too much.