It is possible, but you end up spending 10x as much on the building.
Lots of big cities have grocery stores with parking garages under them, doesn’t seem much different.
Also, the backstock is minimal. Stores are designed for turnaround.
Retail stores are logistics. And part of that is product flow. There are trucks coming in every single day. When you buy an item at a store, that item is deducted from the store's inventory, when that item's stock reaches a certain threshold, an order is immediately placed to the distribution center, and that item is loaded onto a truck and could arrive as soon as that night.
There's no reason to keep anything "in the back" except for high demand items that aren't brought in by a vendor and overflow from items that didn't quite fill a shelf.
Bread/snack cakes (Little Debbie, the bakery also does bread), chips, soda, and liquor/beer are typically handled by vendors. Coca-Cola has a guy come out and stock the Coca-Cola products. Frito-Lay has a guy handle the Frito-Lay products. Etc. They don't work for the store in any capacity.
Vendors typically come during the normal operating hours of the store. Bread guys like to be early in the morning. Chip and soda guys have routes and they'll get to you depending on how the rest of their route goes.
As for other stock, for the grocery side, the distribution center usually palletize stock based on aisle. And the pallets come shrinkwrapped on a truck that arrives at the store between 8 and 10. Someone from the store unloads the pallets from the truck into the warehouse. Once the truck is unloaded, they head back to the distribution center. At the store, the pallets are then staged near their respective aisles and workers restock the shelves overnight.
On the general goods side, the stock is loose in the truck, and a team of people unload the truck and palletize it based on department. Then those pallets are staged in the department for stocking by the overnight crew.
Source: my first job was with WalMart. I worked day stock in a few departments on both the grocery and general goods sides. I worked unloading the trucks on the general goods side. I also worked overnight on the general goods side. I've been involved with a good portion of the store side of the restocking. So all of this information is at least 20 years old, some things may have changed. But I've seen the vendors still while I'm shopping, so the broad strokes likelys till apply.
I assumed it was that and that they are there basically every day. Always notable on the days when a snow storm is announced and bread will be completely wiped out. Only to be fully stocked the next day.
The store is the warehouse and the store owner is allowing self service inside the warehouse but not at checkout.
Having robots means you're automating something that your customers would have done for free. The automation is an additional expense and does not reduce your operating costs.
The online grocery business model only works for two types of customers: those who are willing to pay a premium for convenience and those who need some specialty products that local grocers don't sell.
The first market is a competitor to doordash, this either means automating the in-store pickup or the delivery itself.
The second market is actually a drag on your local grocery stores. You don't want to carry niche products that are only interesting to a tiny portion of your market e.g. products for rare food intolerances, groceries for expats. If you want to carry them in your store, you'd want the customer to preorder them themselves, so you know exactly how much you need and then make them pick them up.
Basically the correct business model is in-house doordash (or B2B doordash) combined with preorders.
Ultimately, I was pointing out why a two story grocery store with a "warehouse" on top doesn't make sense. A place to put stuff is not the issue for retail.
But, what you wrote there is a fair way to look at the core issue for Kroger.
The answer is $. It costs too much and land is cheaper than building up (In most places).
The ones in Bellaire and Meyerland are two level with parking (aka flooding space) below the store and a smaller parking lot on the second level with the store. Bellaire also has a fancy fuel cell setup for some reason. The single level HEB in Montrose(ish) was built into the site of an old complex of charming but nearly abandoned standalone quad/duplexes with many mature oaks. They seem to have retained nearly all of the trees on the grounds in greenspaces within the parking lot and entryway.
Here's some street view of Montrose. They also had a bike air and repair system when it opened. I'm not certain if it stayed in good repair itself. https://maps.app.goo.gl/KxHAvDqKca4E8L8a7