Working with an engine like Unreal Engine for a project of any reasonable size requires working with both hundreds of thousands of active files (my current project's repo's HEAD has ~400k) and hundreds of gigabytes of active files, many of which are many GB on their own. Our repository (in perforce) is currently in the order of 10TB.
Git, even with LFS and partial clone and shallow copies and fsnotify just falls apart at this scale. Add to that the necessity for less-technical people (artists, designers, VFX, audio, etc) to use it, and it is really just a non starter.
I absolutely loathe Perforce (having used and occasionally admin'd it professionally since 2007), but I begrudgingly admit that is is currently the only publicly available VCS that can fulfill all of the requirements of this industry in a practical way. Plastic and the others just aren't there yet.