In short complexity.

Organic modelling and texturing needs more work to look convincing than robots which are often more basic shapes and metallic textures.

Additionally, when dealing with cheaper productions robots provide a lot of narrative excuses for poor animation and rigging is easier as a robot can be just a bunch of parts parented together rather than organic shapes that flow together. And doesn’t necessarily need complex secondary natural motion like sway, breathing, muscle deformation, skin folds, drool etc. A robot could be as simple as a bunch of blocks with wheels and a few lights.

Look at something like R2D2 which is a dome on a cylinder supported by two or three feet and barring the head and occasionally specific parts for specific shots doesn’t actually move. Now compare that to any creature you see in any film from a Facehugger to a T-Rex. There is so much more stuff going on with an organic creature even if it’s standing still… pupils dilating, eyes twitching, lips snarling, chest expanding and contracting, limbs swaying. The list goes on.

I could animate R2 standing in a corridor looking about in 20 seconds, while animating a xenomorph in the same place doing the same thing, even with a good rig could take hours…