The RPi 5 was rolled out differently then previous models. His time they dedicated initial inventory to consumers instead of larger clients and allowed retailers to do pre-orders. They just started shipping and have not fulfilled the backlog of pre-orders yet. Until that happens, you won't see them in stock since they have already been claimed. I pre-ordered mine the first day and my estimated arrival time isn't until some time in November.
The biggest benefit is that I was able to purchase at MSRP and know they will get it to me as soon as earlier orders have been fulfilled. I don't have to constantly refresh the stock finder and try to order before scalpers snatch them up and resell for way above MSRP.