In some ways moving to Unreal is easier, while it does require the devs to learn (or relearn if they’ve used Unreal is past jobs), there is a much wider use base, resources, and documentation then using their own engine.

For smaller sized studios (compared to ones like Ubisoft or EA in terms of employee size) it’s easier to use an engine that someone else builds, as it allows them to focus on making the game, as they don’t have the man power to have entire studios just for building and maintaining in-house engines, or require devs to do double the job of making the game and engine at the same time.