Unreal is much more entrenched than Unity is. At the AAA level, more places hire Unreal devs than Unity devs.

Unity is popular with indies because it's dead simple (Unreal is a complex monster of an engine). But even Unreal doesn't have a monopoly, between things like Source, Lumberyard (which is now FOSS and run by the Linux Foundation), etc. Not to mention you can always roll your own engine, which many places already have.