Unreal and Unity are both rather complex platforms to learn, and wouldn't be very cost-effective for Poser devs to implement at this point, considering the majority of Poser's users are hobbyists with low-end off-the-shelf walmart machines. 

For the more advanced users there is FBX import/export included in the game dev version. FBX exists to port your scenes and animations over to unreal or unity, or whatever other game engine or software package you want to use that is FBX compatible. Back during the release of game dev, Steve Cooper stated that they had been working very closely with Unity developers when implementing FBX. Now, don't you think that if they thought that implementing a real-time game engine as their renderer would be cost-effective and worth the time investment, they would have done so then? I'm sure it's not something they haven't thought about, but for whatever reasons they've chosen not to for now. Plus, Unity, Unreal, etc, are just like Blender and D|S, with constant updates, which would require them to always be updating Poser to keep up with the changes whichever of those engines they wanted to implement were publishing that week. For the number of users that would be interested in it, the balance just isn't there.