I order avocados from instacart: They're all black on the inside when I cut into them.
I order specialty chips or cheeses? They're marked as "Item out of stock," but when I run to the same grocery store that they shopped at after my delivery it's right there in plain sight to me.
I order deli meat from Instacart? They ignore my comments about how I would like it to be sliced and deliver it to me shredded.
Outside of that? I love instacart -- as long as they're packaged goods (pick and ship) and they're not hot deli items or cold/frozen items (I've had drivers put yogurt in the same bag as a lot of frozen veggies and peas and stuff that shouldn't have been frozen was frozen.)
I don't trust anyone to pick out produce. So I have to go buy it. So it's not worth it to me to buy groceries online because I would only be buying non-produce.
I do however buy some things on Amazon that otherwise we might buy in the grocery store, like diapers.
(I'm willing to believe that Instacart + Whole Foods, for example, is a better overall experience--for a premium. But it's not available where I live.)
My speculation is that the reason is some combination of "I want to look at the produce," "I don't want to pay $10 for delivery," "I'm just not organized enough to sit down at my computer and input my order," and "I've always just gone to the grocery store."
Personally, I find Walmart to be a mediocre grocery store that I only go to because it's convenient. Given the other options that already exist for me, I would have pretty much zero interest in getting delivery from Walmart. I'd be using Peapod if I wanted it.