It's because they did this a few weeks back: https://www.theverge.com/2020/8/26/21403245/sony-playstation-5-preorders-invite-only-registration-dualsense-accessories
They allowed tons of people to register and get to them early. Then retailers created the Black Friday-esque rush.
This is all just hype, though. They've made so many, it's not like there's going to be a shortage.
"Sony has confirmed that PS4 games will be playable on the PS5, meaning that the PlayStation 5 is backwards compatible with the PS4. This means that you’ll be able to download games that you’ve purchased from the PlayStation Store on PS4 and play them on the PS5. You’ll also be able to insert physical PS4 discs into the PS5 and play them on the next-gen console. You won't be able to use physical media on the PS5 Digital Edition because it doesn't have a disc drive..."
https://www.pushsquare.com/guides/ps5-backwards-compatibility-can-you-play-ps4-games-on-playstation-5