I'm guessing because engines like Unreal have game developers in the bag. VFX, on the other hand, has been a long term pursuit of Unreal and can have a large payoff for them. Based on some second hand insider knowledge I have, the VFX and animation industries are all over the place in terms of tooling, and a lot of pipelines are more or less bass-ackwards, especially when it comes to the pre-visualization stage of production. Even big studios are relying on tools that would appear "legacy" to many software engineers and are inferior to game engines in a lot of ways. The status quo often remains because these studios are already making enough money and have little incentive to address their internal bureaucracy that prevents them from making legitimate improvements to their pipelines. There are plenty of technical directors and artists out there pushing to use things like Unreal Engine, but these efforts often go nowhere for a variety of reasons. I know of at least one major animation studio that pondered over utilizing Unreal for
over a decade, and there's another major studio I'm aware of where they're trying to use Unreal but the organizational support just isn't there.
Another problem I see in adoption of Unreal in animation is that the entertainment industry historically hasn't valued its engineers in the same way that Silicon Valley does. Unless you're working directly for a company like Unreal or Autodesk, why would a software engineer go work on some shitty duct-taped pipeline for a studio that will inevitably pay them less and make them work more hours, unless said engineer is particularly enamored with movies or shows? Based on what I know, I believe that the entertainment industry struggles to find and hold on to engineering talent, and not in the cliche way we hear from every company out there.
However, if Unreal reaches the same level of ubiquity and vendor lock-in within VFX that Autodesk has, for instance, then it would be a humongous cash cow for them. They want to keep their engine free to that industry for that reason, and it's a big incentive considering the exorbitant costs typical to VFX software.