1) Amazon has observed a massive influx of orders and fulfilling centralized orders requires a large amount of workers in fulfillment centers – Amazon is hiring more than 100,000 workers for this reason[1] – and getting those orders in and out of those warehouses takes a good chunk of logistics planning and time. Independent retailers don't need to deal with this, particularly if they're shipping direct or use their own facilities.
2) They use their own delivery networks for a majority of deliveries in cities these days to bypass sending via the normal postage system. The platform, Amazon Flex[2], saves them money on postage but likely needs to ramp up scale–Flex usually actually means deliveries are faster, but as order volume scales up, it'll slow down deliveries.
Amazon is bottlenecked on both fronts: a massive influx of orders means needing to get those products into its warehouses, then actually having workers fulfill them, and it needs to manage the shipping side, due to its reliance on Amazon Flex in city areas. Independent retailers just aren't bottlenecked like this – they have their own set of inventory that isn't super broad, and they ship with normal delivery networks, which are generally delivering on time in North America right now.
[1] https://blog.aboutamazon.com/operations/amazon-opening-10000...
The same way everyone does when trying to solve this problem systemically (vs individually), by making imperfect decisions and leaning on categories.
My UVC lamp was delayed. I think that's more important than someone else's "third month" bunker food, but what are you going to do.