COVID has thrown the world upside down as far as deliveries and from what friends and I've put together there are several key reasons.
Early on, there was a problem with nobody working so nothing was being produced. That seems to be pretty well behind us.
The wholesalers had stock in inventory and not knowing where the economy was headed they really did not want to put in new orders to manufacture in lot-size quantities. The best example I know personally is the local manager of a West Marine. I could normally walk into almost any of their stores to buy stainless fittings, but suddenly needed to wait for parts from the other side of the country as the company used up stock. He said they seem to be coming out of that but it really depends on the part in whether it will be on hand, coming from he other coast, or they just can't tell him when.
He added that because they buy parts by the thousands and it ties up so much inventory cost, they need to be fairly sure the stuff will sell fast enough to commit the money. On top of that, they need to raise prices on a lot of items that they do order because of tariffs imposed by the current Administration, which are a back tax on US and they've seen higher prices decrease demand on tariffed items.
The shippers are still totally screwed up too. Early on, they were giving priority to PPE and we all could understand that, but normal overnight stuff suddenly took forever. Now, it appears the problem is more with EVERYBODY getting used to home delivery and so the shippers are still overwhelmed. I went to replace a broken Fitbit this week and there is clearly no shortage on them, as I could walk out with one from almost any Target, Walmart, or Best Buy, but the nearest possible delivery from Amazon was two weeks out.