Fine leathers actually have a really bad drop rate, and as a farmable, its really annoying to get. In the past to get thousands of taps, you'd need thousands of wood boards, and finest leathers, which are both really irritating to farm, and craft / acquire.
Coincidentally, these are annoying, simple mats people acquire over time. However, events always spit out quite a lot of differing amounts item archetypes, like pet mats (people don't actively farm these anymore like I did) I farmed around 150-200m worth of pet material items when it first came out.
However, now that A erg got realeased, fine leather has been on fire and has been exploding as of late, and with more people wanting to go for A erg for cheaper access, its just going to get worse. Fine leather is LOCKED to advanced shadow missions and similar dungeons of similar difficulty, and poor rng from Ingredient hunting.
Life materials in boxes and fishing boxes literally balances everything out. Silks are simple to make NOW with refine update. Just afk near a spider farm, and make something to eat and boom you'll have a ton of cobwebs ~> thin thread ~> silk. These items are so simple to make now, that getting them in excess is natural nowadays. FINE FABRIC USED TO BE RNG, and it was a terrible erg step for so many people, and most people got their pity taps from event boxes.