I chanced upon this question at 5:11 of this YouTube video. Their answer is "no idea".
A cynic would answer profit motive, for it costs money to add safety pads to all these carts and airlines aren't profiting especially in this COVID19 pandemic.
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Because an inch or two of rubber isn’t going to stop a 300lb cart from breaking your foot if it goes out of control down an aisle. If your elbow is in the wrong position, you could hit it with a 300lb roll of memory foam and cause a compound fracture.
The maker of this video is just looking for things to complain about. Keep your body parts out of the aisle and you’ll be fine.