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why do many titles switch from their proprietary engine to unreal engine

For example, According to List of Unreal Engine games - Wikipedia

Dragon Quest XI
Dragon Ball FighterZ
Darksiders III
Street Fighter V
...

They don’t switch.

For each of these titles, it was likely determined that it was cheaper and more effective to license an existing engine and toolset compared to the cost of upgrading or replacing the old one to suit a next-generation hardware platform.