That's mainly for 2 reasons:
- people conciously picking glamour over other goals because they simply enjoy it the most
(and since FFXIV is a beautiful game with amazing glamours, it is not unlikely a lot of people who put much emphasis on these things have gathered here)- fluff being an actual long-time reward.
If I spend 300 runs on getting this pony/bird/whatever, I have something that will stay with me forever.
In 2 years, it will still look amazing, it will still perfectly coordinate with the color of my outfit and still carry me wherever I need to go.
But why would I spend 300 runs on farming a weapon just for the stats when those stats are gonna be outdated in a few weeks?
Personally, I don't exactly mind this lack of longevity of gear, since I already have moved on to considering fluff the true goal, but a more horizontal gear progression would accomodate those who do not care about fluff without taking away anything the fluff lovers care about. But there are many things in the way of this - meaningful different stats would be needed on gear to make different sets really necessary for different content, and that would not just require REALLY careful finetuning but also a complete overhaul of the system since the current limits to how much gear one can hold are not even remotely adequate for such a progression form.
They are events. If event items were anything but fluff we'd have a real problem on our hands.
If someone misses an event now, it's like "well, it's just fluff, but if you really care about it, you can buy it in the cs".
If event gear was cutting-edge stuff or event mounts were faster than regular mounts or event furniture had special gameplay functions, we'd be right at the point where you can literally BUY progress and advantages in the CS.
And if they weren't availabe in the cash shop but forever lost, that would alienate a big part of the community and be even worse.
The problem with grinding Fates for fluff for events is the "grinding FATEs" part, not the "fluff rewards" one.