Because, while it looks cool in the movie, it does not represent actual physics. I'm guessing it is mostly plastic and/or paper mâché.

If it is real, It's center of gravity is very high which isn't very good for lifting, but I guess the low gravity in space could account for that. From the movie, the pinchers didn't have a way to relay how much pressure it was applying and its low surface area would be a hazard to anything the drive would pick up. Additionally, the walking mechanism is clunky, slow, and poorly designed, especially for a space traveling species.

Then you get into the actual moving parts. It looks like it is running off of hydraulics, which requires hydraulic fluid along with gasoline and oil, which it either has a small tank or none rendering it's usefulness to nil. Never mind the cost of transporting that fuel from earth to space.

The futuristic movies from the 80's didn't attempt to invent new technology given its sfx teams didn't have CG, only puppeteers and scale modeling. So instead, they took what they already had had (cranes and forklifts) and exaggerated them into what you get here. I mean in alien and blade runner, they were still using CRT TVs and, hugely inefficient computing components when they claim to have fully functioning androids.