Others answered but it's easier/cheaper for them to use a vendor's engine. It makes sense.
What sucks is that UE seems to almost have a monopoly on engine leasing. I wish there were more options. Having all games use the same engine is putting too many eggs in the same basket.
They claimed that it was expensive and was part of the reason for cyberpunk's turbulent launch.
It's a real shame though, most UE5 runs awful it seems, and are still limited by single thread performance, unlike RED Engine which scales far better with more CPU cores.
Tbh RED engine also has its plethora of problems, missing features, and makes it harder to onboard new team members (need to train on new engine instead of basically every single dev having experience with unity or unreal).