The short answer is that they gambled and they lost.
The long answer is that releasing a high budget, triple-A title across multiple platforms isn't done in isolation. They were busy coordinating advertising campaigns, partnership deals, distribution, marketing, and other time-sensitive external processes.
Many of those external contracts were likely time sensitive, perhaps with financial penalties for missing the ship date. For example, did you know that big box retail stores don't put up giant advertisements or end caps for new products for free? They actually charge vendors to occupy prime space on the retail floor and put up product advertisements within their stores. In this case, stores like Best Buy even had Cyberpunk 2077 ads plastered on the outside of the store windows with a specific release date, which they missed once already. Each time a release window comes and goes without a product to sell, the company loses huge amounts of money to these contractual obligations.
Also, building these games is expensive. If they secured external financing to pay the huge development teams, that money comes with conditions about when they must start paying it back. No product = no way to pay, unless you go out and secure emergency loans to pay off your other financing, which is about as expensive as you would imagine.
These things aren't planned last minute. The launch date window was likely determined over a year ago, with huge amounts of money and work going into those specific dates.
Really, this is a failure of project and product management. The teams should have realized that they weren't on track to reach the launch dates and narrowed their scope early on. It looks like they overestimated how much they could accomplish and assumed they could fit it all in to the allotted time, which was obviously not the case.