I can only speak about the Impressionist Tarot and the kit card stock was definitely thicker and (IMO) of better quality than the non-kit cards. I also have an older and more recent version of Nefertari's Tarots and the earlier edition (with black backs) is much better quality than the more recent (double eye of Horus back). The recent edition is thinner and the edges are already fraying. As to the Book of Shadows (kit), I find the card stock thin and very unimpressive. Glad I didn't pay what the poor old Kickstarter backers paid for it.
KICKSTARTER for Book of Shadows ? Since when ?
The issue raised was the difference between cards in kits and cards on their own. IN the case of the Impressionist - I do concede that the kit cards are a little thicker - but honestly, IMHO, not enough to get excited about - the deck only ones seem perfectly fine to me, and I would never have noticed if this thread hadn't made me get them out.
Now, that the standard is slipping - I would agree up to a point - especially in the case of Llewellyn, whose card stock really is thin and not that durable. LoS stock may or not be thinner than it used to be - but I haven't had any issues with durability. None of mine - old or new - is fraying at all. USGames I haven't really thought about; it's so long since there was a new deck to think about - except the "in-a-tin" ones, which are small enough that it makes no real difference ! - and I don't tend to buy new prints of decks I already own. The Omegaland one was fine.
Meanwhile playing card stock (Bicycle and USPC) is fantastic; strong, linen finish, lovely to work with. Why Tarot card publishers are using inferior stock is anyone's guess but the cynic in me has some ideas.
Can't actually think of any myself except that tarot cards DO have a rather smaller market for each new one brought out, where playing cards go on for ever.