To piggyback on this for the unfamiliar... most retailers with both online and brick & mortar storefronts combine inventory for the two operations. This is why you go to HomeDepot.com, look at a hammer, and it says there are 7 in stock at your local store. With most retail sale goods this isn't a problem.
High-demand products that sell easily online present a challenge of what happens when you have 10 items, put 10 on the shelf, and they start selling online concurrent with people picking them off the shelf. You sell all 10 of your online items in the first 90 minutes, but in that time four people have picked items up off the shelf and took them to a register. Realize that most large retailers have a goal of having an item picked from its shelf location within 1-2 hours of the time a customer places an order. Also realize that the way that inventory management systems work it may take several hours from the time a unit is purchased at a register, it may be several hours or overnight before the count is changed online.
All things here add up to a system where you a parade of customers who come into the shore go into enraged screaming tirades because they bought the high-demand item online and when they reach the store an hour later, not only is the product not in stock after all but an employee hadn't even gone to pull the item yet, much less discover they don't have the item and call the customer before they come in.
Let's also account for the fact that a new video game console is going to be a major problem for store shrink (lost merchandise). One unit gets shoplifted, that might be enough to account for 2/3 of your local Target's daily allotment for shrink. That's before you account for other items being shoplifted, product being incorrectly logged on the shelf or in receiving, products not being scanned properly at the register, etc. Keeping the products in receiving and letting them sell online eliminates a lot of this and let's face it, the product still sells.
Retailers also have basically no incentive to sell to consumers rather than scalpers. Sony wants every PS5 in the hands of a user, not sitting unused in a scalper's garage. That's not technically a problem for them if scalpers sell all the stock they buy, but I've read a bunch of articles that make it clear this isn't happening. By contrast, the store manager at your local Target doesn't care if the PS5 is purchased by a family or by a scalper. S/he just cares about whether the store moved product at full price and got paid for it.