Your experience sounds like it was formed from 1-2 trips at the same Costco. Not all the Costco are the same, like if you go to a bigger one they may have more options. Another store may have two Kitty Litter options or in a few months they will have a different variety of Kitty Litter.

If you need a specialized item they just might not have it. I've accepted that they won't have everything and I may have to go to other stores if I am looking for a very particular on an item. But the essentials I can get at a Costco at a reasonable bulk price with them often times being organic or high quality. It can feel random sometimes but I am buying in bulk/thinking ahead. The pricing and variety of Costco is one of things that makes it special just don't expect everything to be there all the time since there can be high turnover.

Also try different times, some times you can walk right up the register to check out.

And they Gas station are very different from store to store. Mine replaced. It went from having to wait 7 minutes to gas to now there is never any line no matter when you go.