Why can't cats walk backwards?

They can walk backwards, they just usually don't. Most of the time, if they do walk backwards, it is because they are trying to get out of something or get something off of them.

I find an example of a cat walking backwards:

https://www.youtube.com/watch?v=NLCXKN2X4zM

Generally cats might walk backwards to get out of somewhere (like a narrow crawlspace) or because they're simply lazy as hell. We had a television on top of a cabinet where there was a clearance of about two inches or so between the television screen and the edge of the top of the cabinet, and the cats liked to walk across that little strip. Well, at one point I stood nearby while a cat was en route to the other side. He couldn't turn around and didn't feel like jumping down just to jump back up for scratches, so he kinda scooted back an inch at a time and it was the funniest thing I'd seen in weeks. He just put in so much effort just for cuddles. This was a carefully calculated operation. So walking backwards isn't always a sign of distress - sometimes they're just excited.

They can. Sometimes I'll stand over my cat with her between my legs, bend over her and give her a look, sometimes if she doesn't see me see gets scared a backs away by walking backwards and then turning around and running off.

They can - I've seen them do it. for a few steps.

They can but very retardely