40 for a pen seems excessive for a regular budget even if 40 is not a lot of money anymore

Yes, like the GP says, it's a luxury pen. They didn't buy it for purely utilitarian reasons. They might like that it has an all-metal construction or that it weighs 88g. They might prefer the ink formulation used for the cartridges. They might like how the pen works in their preferred notebook. For many people, tactility (and other sensations too) is just as, if not, more important than, utility.

Only if this pen were a daily driver and not just kept in a case because, well, it’s expensive and we collect stuff..

Its a totally usable pen everyday if you want. I don't think most people spending $40 on a pen put it in a display case, its not a $4000 pen.