Most other major stores have Pokéstops so I don’t get why we can’t have any for Costco buildings. They even sell Pokémon stuff (mainly books in my local area) in many locations which makes it more confusing.
This is going to sound strange to you, but Wayfarer doesn’t really have anything to do with Pokemon. It is a map or database of wayspots for things that are great places to be social, exercise, or explore. You don’t do any of those things at a Costco. You can socialize at their picnic tables, though that’s a little bit of a stretch, IMO. The things that are approved through Wayfarer serve 3 different games - the other 2 have no connection to Pokemon.
Welcome to the forum @Coolmann225
Large retail chains like Costco would normally not be eligible as @seaprincesshnb says. Some of them do have Sponsored locations, which may be what you are seeing. In those cases, a company pays the Pokémon Go devs to have a list of specific locations promoted in game as Pokéstops or Gyms. Sponsored locations are not actual wayspots in the wayfarer database though.
Costco isn’t a participant in the sponsored program. However most of the 18 or so locations I’ve been to, since I started playing that is… have a single stop for the food counter/tables area, and one has one for the pharmacy counter. Only one has one for the store itself, and that one is (arguably) their oldest location. (It was originally a Price Club, the two chains merged in the 90s, and PC was older than Costco.)
(And no, despite my fandom… none of those were me.)
My local Costco has a wayspot from a “Price Club” plaque that was dedicated years ago. A different Costco I visited this summer had employee lockers in a public area, and one of them was converted to a trinket box. You might have a better chance of making a stop if you narrow it down to one thing in the store.