The amount of 3D polygons necessary to achieve a comparable level of fidelity to 2D sprites would very quickly exceed the limit of smartphones (and even if it didn’t, it would kill their battery life). Remember that there can be over 100 chickens on-screen at any given moment.
@Sammarald: Correct. UVE is optimized for sprites, not 3D models. Although, fun fact: the planets in CIU are 2D. Very deceptive 2D, but 2D nonetheless 
IIRC the only pseudo-3D things in the entire series are some CI5 environments, the CI5 logo, and the portals/wormholes.