Over here in the world we call reality, Costco is substantially cheaper on meat/produce and beans/rice, which is a large majority of what my family eats. And when they do have another product I want to buy, it's usually medium-high quality, and a very good price. The one thing you state as a benefit isn't at all, since Costco actually is remarkably inconvenient for one stop shopping. Costco carries less than 10% of the items that a normal grocery store does, averaging about 4000 skus per store. Whole foods, as you're probably aware, carries between 40-49k skus per store. Costco is awesome for staples. If you want some fresh thyme, some tomatillo salsa, or ginger root you're not going to be able to get it there.
If I run to a normal grocery store like the WF they are putting in down the street from my house, I make one stop and I'm done. If I go to Costco, I usually need to stop elsewhere on the way home to get everything I need. But I like savings the moneys, so I do it.
I don't shop at Costco for the things that I'm very particular about, mostly because you'll rarely find high end stuff there. I'm a bit of a cooking nerd, so my knives, my espresso machine, my pots and pans all came from elsewhere. But for things I'm not particular about, and I just want a good price on a quality item that will last for a long time (and if it doesn't, you walk it back in and they give you your money back no questions asked), I love Costco.
Have you ever shopped at a Costco? It seems like you don't understand it very well.