A big reason was stability.
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During today's "State of Unreal" stream, CD Projekt Red had a moment to explain just why they have decided to use Unreal Engine 5 for their upcoming Witcher sequel. The decision was made a while back to develop this sequel, be it called The Witcher 4 or something else, using Unreal Engine 5 instead of their in-house REDengine 4. I bet you didn't see this move coming, but if you are a betting man (or woman) be sure to check out casino online.

CD Projekt Red's CTO Paweł Zawodny said "the shift towards open world support that brought Unreal Engine 5 to our attention." He continues, "This opens a new chapter for us where we really want to see how our experience in building open-world games gets combined with all the engineering power of Epic."

Game director Jason Slama added that open-world games need a stable environment in order to function properly.

"One of the things that is really important to keep in mind when talking about open-world games versus, let’s say, linear games is the possibilities of the things that can go wrong or the scenarios that you have to consider are exponentially higher than linear games. Players can go in whatever direction they want, they can handle content in any order that they want theoretically, and to really encapsulate that means that you need a really stable environment where you can be able to make changes with a high level of confidence that it’s not going to break in 1,600 other places down the line."
The Witcher 4 has very few details available for it at present. However, we do know that there will be a new Witcher school added in the new game, one that has never been included in The Witcher franchise before.