They're serving different market segments. Costco literally charges an admission fee. "Pay me money to come spend more money inside my store and it'll be really cool, promise" is the antithesis of the Walmart philosophy. Walmart itself recognizes this, which is why Sam's Club exists (no reason to let a competitor run away with that less price-sensitive market segment without a fight, of course).

For any arbitrary shopper-item combination, as long as the item can fit the basic merchantability requirements over the duration of the return period, the only significant criterion is going to be price. Knowing this and then ensuring that they have the cheapest merchantable items in every product class is how Walmart became the de-facto shopping nexus everywhere that major social and/or governmental interference hasn't stopped them. It's the place to go to get the stuff you have to buy, not the stuff you want to buy.

If your standards are too high to wear a shirt that consists of 80% recycled plastic, you don't buy shirts at Walmart. But that doesn't mean you automatically have the same high standards for a garden spade, a toilet seat, or a kitchen spatula. If we imagine that we need all four of these items today, we go to Walmart to buy the latter three and then use the money saved to invest in a better 100% cotton shirt from $FAVORITE_SHIRT_VENDOR.

I have purchased clothes, shoes, and just about every other household item at Walmart at some point in my life. The quality is no different from buying things in the same price range at any store, in person or online. Often, the stores are selling the exact same thing from the same manufacturers.